basicly it turns it off because your gpu needs all extra memory it can get and also it is extra work for you're card as well being will all the full animation
so the vid card adjust to help it work more efficiant depending on what you are playing if you notice certain games lag if you have full win animations on ?
or if the virus protect is enabled
so dont worry about it happenes to me on occasion when i am doing stuff but as long as everything else works fine it is nothing to worrry about also make sure you dont have programs that can change the way your desktop looks that also can be messing with the windows classic and full aero