Congratulations - you're the first Gamestudio user who finally discovered that all entity angles are wrong!
Seriously: In three-dimensional space angles have 3 values instead of one, meaning that you rotate the object three times about one of its axes. Rotations are not commutative, so you need a system to define the order of the 3 rotations, and which axes to use. This is the famous "Euler angles" system, i.e. pan, tilt, and roll. All spatial rotations, in math as well as in game engines, normally use those 3 Euler angles.
It's not this hard to understand but a little hidden in the manual (under "Syntax / Variables"). I've put it also in the entity description now:
http://manual.conitec.net/aentity-pan.htm