Can someone shed some light on DivX GPU useage for me while using FF & ATI GPU?

[Udgnim]

vid card is a 4870 512. I'm loading a DivX video in FF. when the FF window is open, the GPU usage is around 1-2%. when the FF window is minimized, the GPU usage shoots up to 10-15%.

the video is just loading and not being played. can anyone explain why this happens? thanks