That is interesting. I have not used the aspect ratio's in a while, so never noticed it.
I took a Xvid movie, paused it. When I try different aspect ratio's, it is not stretching, but simply cropping off the top/bottom/left/right ect. Similar to the typical "Pan & Scant" (This movie has been modified to fir your screen),
I do not know when this started, but I am definitely duplicating what you are seeing here.