it's not a bug, it's a feature
Seriously, the predicting engine in the current version can't do a dead-on lock on the endsize since it's to depended on several other things in the re-compression engine.
So sometimes a movie comes out a bit longer then expected, and there's no real other solution then just setting the size a bit lower and re-doing the movie.
The prediction engine we are developing at this moment will be more acurate in predicting the end size. But this engine is very very unstable at this moment. I expect it to be build in the 1.2.x version of DVD2one.