All I knew that the vanilla version had a limited level cap - level 10. However my ranger has just leveled up to level 11. How is it possible?

I did not install any mods, I am just playing the GOG version as it is.

As far as I remember, the GOG version does include a very old version of a community patch which -- among presumably more crucial fixes -- increased the level cap. (This same .DLL was apparently a cornerstone of what became the Circle of 8 modpack.) Presumably, GOG had to include this to even get the game to work reliably, as they usually don't use third-party patches or mods that add or change content otherwise, unless the current publisher is keen on it.

That seems borderline without any co8 credits, not to mention the false information it spread on what is sold.

I did a quick search on internet and co8 seems have been released rather fast, even before atari patch 2. so some confusions are possible, otherwise it would be weird. For example imagine an Atari pressure so there's no co8 credits would be weird.

The case looks bizarre.