That is apparently what AAC + VBR does. Lets say you choose 192 Kbit AAC + VBR. What VBR _should_ do is analyse the complete song, then encode hard parts with more than 192 KBit, easy parts with less than 192 KBit, aiming at an average of exactly 192 KBit (this would make it sound slightly better than 192 KBit fixed and use exactly the same space, using a bit more encoding time). Apparently it just encodes to _at least_ 192 KBit. That way, all songs will come out in exactly the same quality, no matter whether they were hard or easy to encode, but the hard ones take more space. The number displayed could be the average bitrate for that song, so each will have a different number.
Any tracks that were imported that version of iTunes will continue to display the varied bitrates. The only solution is to re-import them with a newer version of iTunes. 🙂