The documentation says that Math.atan returns values between 0 and pi, but testing shows that it actually returns values between -(pi)/2 and pi/2. I prefer the behavior that it has now since it matches the usual mathematical convention, but I just wanted to let you know the documentation is incorrect.
Thanks for the report. I'd guess the documentation is simply wrong, but I'm not sure what the intended design is for that method. I'll have this checked and corrected.