SDPanthom wrote an explanation on that here aswell:
http://www.wowinterface.com/forums/s...7&postcount=10
Regarding your fix. Does the function only has to be called once per texture prior to any SetRotation calls?
If you have to call it after every SetRotation you will be screwed. What if you rotate the texture in a way that your math. prefixed (+-) are wrong because points have switched places.