it may be simple physics
I believe the latter to be true, though I can't necessarily explain the mathematics behind it. George G's earlier message paralleling this to crown molding cuts seems to offer the best explanation.
A miter is made in a vertical plane 90 degrees to the table. When you add a bevel this changes the approach angle of the blade to other than vertical. I can see how this would change the length of the mitered angle as well.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 09-05-2013 at 08:29 AM.