So the version number on the Terraria download page is 2.0 but the version number on the actual game is 1.2.4.1.

Shouldn't the two be the same?

This number is the installer package version, not the game version. And, yes, it's confusing and not very logical. :/

I gotcha. Yes, still a bit confusing. Especially for newer players, I would think.

Yes and people have asked for years that they put information of both versions on each game in the library yet they never do..

Well, the numbering is logical like any versioning system but it's not that useful to us, the users, and having two versioning systems is bloody confusing. Mac and Linux builds having their own version numbers adds further confusion (and witcher 2 for windows has it's own version being only 3.x.x.x game I know on GOG).

We do have to bear in mind that GOG does fix games and update the installers on it's own too. When they do, they do not or can not (no source code access) update the game version. Also when they update the installers themselves, like when they updated them from 1.x.x.x to 2.x.x.x, they do not change the actual game files, only the installer and install scripts. But those changes can be important too as they may improve compatibility with specific system (as, iirc, 2.x.x.x did with win 8), update the programs or wrappers that are used to run the game (DOSBox, ScummVM, Wine, glide wrapper among others) or update their settings for better stability, performance or compatibility. On the other hand, some GOG changes do not really matter to most of us, like when they changed the adverts in the installers.

So maybe GOG should have both the game version and their internal version in the installers? Like Terraria installer would be setup_terraria_1.2.4.1_2.0.0.1 instead of just setup_terraria_2.0.0.1 or something?