it seems to me there's a bug in the texture coordinate assignments for the edgeFile graphics that are assigned via a SetBackdrop function call.
the effect is most pronounced when a frame is sized down very small -- the left/right/top/bottom textures stretch and distort non-linearly as the size changes.
I see what you're talking about, but I'm afraid this isn't unintended behavior.
The backdrop system wasn't designed with the resizing of high detail borders in mind, and while we might improve it at some point in time, it's not likely to be a high priority item.