I’ve never seen that myself, but I would guess your original video is 16×9 and your exporting to 4×3 and it is asking a basic “pan-and-scan” question (do you want me to use the left, right or center of the video to export so that I can fit into the aspect you’ve chosen?)
Again, I didn’t even know it did this as when I’ve been in similar situations, it just squeezes my 16×9 into a 4×3 box, which is very annoying.
Does this sound logical?
Give us some more info and we may be able to help better (timeline settings and such).
How are you exporting (quicktime conversion)?