[MEncoder-users] different xvid bitrate, same size for resulting file ??

[Uzu Maki]

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nice -n 3 mencoder -oac mp3lame -lameopts abr:br=128 -aid 129 -ovc
xvid -xvidencopts bitrate=1350:pass=1 -vf
crop=720:560:0:8,scale=620:340,pp=lb dvd://1 -o /dev/null
nice -n 3 mencoder -oac mp3lame -lameopts abr:br=128 -aid 129 -ovc
xvid -xvidencopts bitrate=1350:pass=2 -vf
crop=720:560:0:8,scale=620:340,pp=lb dvd://1 -o "movie1350.avi"
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as you can see I used bitrate = 1350 and got this:
571276936 2006-01-03 21:47 movie1350.avi (545Mo)

before that, I used: 1250 and got the same file...

how can this be ?
thanx all and happy new year
=)