The Galaxy client shows My Collection as having 1 fewer game than the website shows. Both have the same number of pages and both have 17 items on the last page.

Is there a specific game I might have that is known to not get counted by the client?

Strange,but I see only 16 in your library.On second count there is now 15.

Some games are somehow not compatible with Galaxy client (as in you cannot download and install it from the client, though you still can do it manually from your account page). In my case it's SiN Gold.

SiN Gold is in my library when using Galaxy. It's just the install button that's grayed out. You can still access the offline installers and extras through Galaxy. The Number of games showed next to My Collection is the same on the website and Galaxy, even though I have at least 3 games which aren't compatible with Galaxy.

Is it possible that the game is under "Hidden" catagory (instead of "My Collection")?

Nope, I have nothing under hidden. This type of game still seems to be counted. I have a half-dozen of these and they seem to show up both in a browser and in Galaxy equally (other than being Galaxy-incompatible.)

And you don't have any filter on?

So the client shows all the games, but counts one less that it shows?

That's how it appears.

I was hoping someone might have seen it before or maybe knew if some specific game causes it.

That happens to me, but from what you are describing it's probably for a different reason for you. I'll explain what's happening for me anyway just in case.

I'm using Galaxy on macOS and so by default my library shows only the macOS compatible games whenever I start viewing my library. AFAICT, there is no way to set a different default (beginning point for search), but I'm fine with that. This initial system selection is indicated by the "SYSTEM" menu in the search bar having an "x" instead of a downward arrow "v". Clicking on the menu itself shows "OSX" selected and allows changing the selection. OTOH, simply clicking on the "x" removes all selections from the "SYSTEM" menu without having to open it and then reveals all of my non-hidden games for all systems. I happen to have a few games not compatible with macOS because I can get them to work in WINE.

There is a heading below the search bar just above the grid layout of games named "MY COLLECTION", and it has a number in parentheses which accurately counts the complete listing of non-hidden games for only my system. Removing the "SYSTEM" selections reveals all my games but that does not change this number. That means the number does not count all my games, and it does not count all the games shown, but instead counts only the games compatible with my system that are not hidden. It seems like no matter what search options I choose (genre, system, language, etc.), that number in parentheses stays constant. It apparently represents all non-hidden games compatible with my system by which I can compare the results from the search.

The "MY COLLECTION" heading is actually a menu that also has a "HIDDEN" option. The "HIDDEN" option has a number in parentheses which accurately counts all the games I have hidden regardless of compatibility. That's different from the "MY COLLECTION" number that counts only compatible games. When "HIDDEN" is selected the grid shows only the games for my system (macOS), and that of course is less than the total number of hidden games (in parenthesis). By removing all the "SYSTEM" selections the "HIDDEN" listing will show all my hidden games. The number in parentheses for "HIDDEN" does not change, but it then accurately represents what is shown.

To clarify, both of the numbers in parentheses after "MY COLLECTION" and "HIDDEN" remain constant regardless of the search settings.

Recall I stated the default search starts with "SYSTEM" set to my system (macOS). Removing that condition (such as clicking its "x") causes a "CLEAR FILTERS x" item to appear next to the "MY COLLECTION / HIDDEN" menu. Apparently changing the default "SYSTEM" selections in any way from the default (of all versions of macOS) is considered to be a filter. That means even if one those items is unselected then that will be considered as a filter. Obviously selecting anything else in any of the search menus will also be considered as a filter.

In conclusion, this means the "MY COLLECTION" number plus the "HIDDEN" number is not the total of all my games, because the "MY COLLECTION" number counts only the games compatible with my system and I have a few games that are not compatible with macOS. OTOH, all my games can be revealed by simply removing all selections from all the search menus. However, doing that will reveal the "CLEAR FILTERS x" option which I consider a little bit misleading because selecting "CLEAR FILTERS" actually chooses the default system filters instead of clearing everything. And that means all my games won't be shown in that case.


EDIT:
From all of that, I would presume that if I hid all of my games then the "HIDDEN" number would be the total of all my games, because the "HIDDEN" number counts all hidden games regardless of compatibility. Whereas if I were to unhide all of my games then I would expect the "MY COLLECTION" number would be even less than the current total of both numbers, because there are some hidden games that are not compatible and so wouldn't be counted by the "MY COLLECTION" number.

All in all, this seems different than the website. I suspect that's because the website doesn't base it's counting on the operating system. That makes sense to me because the website is viewed with a web browser, and web browsers don't have to accurately reveal the operating system.


EDIT 2:
Since you don't have any games in the "HIDDEN" category, I'd say check to see which games are not compatible with your operating system by reviewing the search results for each of the three: WINDOWS, OSX, LINUX. Though, it seems to me everything on GOG is listed for "WINDOWS"; the total number of search results from the store match the total number of games in the store. So your library might not reveal any difference on the website. You may need to narrow down to the exact operating system versions one by one and see if there is a difference within Galaxy on your computer.

I noticed for incompatible games there is the phrase "Not compatible" in thin red lettering in the lower corner of a game in grid view, or at the far right side of a game in list view, unless the mouse cursor is over the game in which case a square with a "v" appears in those locations (effectively replacing that phrase) as an offering of a menu for the game.

Thanks. I somehow missed that they had opened a new thread.

Edit: As posted in that thread by Blue Johny., The solution was:

Try clicking on the "Library" -> "All my games" again as it will refresh the library view.

see: https://www.gog.com/forum/general/gog_galaxy_official_discussion_thread/post290

It's good to know that your problem has been solved. :)