iPod bitrate problem-help!

Hi everyone. I’m using DVDFab to rip DVDs to my ipod. I put the setting on mpeg-4 (not h264), resolution 640x352, and a bitrate of 2500. But the problem is that it doesn’t seem to want to rip at that bitrate. When I imported the file into iTunes, it said that the bitrate was about 1500-1600. This has actually happened with a few DVDs. Is there something wrong with my settings? I have a 30GB iPod and I want to rip at the best bitrate and frame size possible because I sometimes watch these movies on the TV. I believe that using mpeg4 it is possible to rip at up to 2500 with a resolution of 640x420. Any input is greatly appreciated. Thanks.