it seems you're right... strange.
but now i remember that in pak-9's programming tutorial
he says that atan2 comes with some additional features like avoiding division by zero and some wrapping around the "ends" of a circle [you know, between r=0 and r=2*$PI]
/looks into the guide
hm can't find it now, maybe he told me on irc one time, i don't know.
when i use atan instead of atan2 in a movement there are some glitches to be found but they really are minor. and there's a small dot in the middle of the screen. plus it's slightly slower than atan2.
so i think it might actually be sometimes faster to use atan(a/b) than atan2(a,b) in an ssc but i'll stick to atan2 in movements.
but have a look [it's all in the movement]: