This thread and this thread at Apple Support Discussions describes a bug in QuickTime 7.3 which ignores your iTunes VBR (variable bit rate) settings for importing or converting AAC tracks. As a result, you always get VBR. (Thanks to Correspondent Christian who pointed this out to me when describing a problem with the "Pass-Through" setting of Join Together. This option will only be available if your selected tracks all have the same bit and sample rate. Of course, if they are VBR, they won't all have the same bit rate and Pass-Through won't be available.)
I smell iTunes 7.5.1 and/or QuickTime 7.3.1 coming soon. At least, I hope that's what I smell.