I'm rendering a short video to .mp4.  When I do that I get a black edge around the video.  If I render the same project to .wmv, then there is no edge. 
The video is a non standard size of 1000x504.
Any idea why this could be?

Yes, I can render to .wmv and then convert, but I would like to understand why this occurs to try and avoid it.

Thanks

Les