Wrong total video bitrate in mkv to dvd

[bucho.de.q]

Hi everybody, I've got some trouble concerning with settings about the bitrate inside the converted file dvd from a mkv h.264, I need to set 7700kbit as max and 6000 as mid bitrates on video and final size 6,0GB, after process the converted dvd has a wrong max bitrate (about 9200)and size 5,30GB, of course it's completely different from that I've chosen. Why?? I think this is a bug.