Basically, MM seems to be copying the bitrate of the currently playing track and displaying it for the previously playing track. Observe:
It's quite easy to reproduce, though I have only tested it with a mix of VBR and CBR mp3 files.
Just enable the bitrate column and queue up some mp3's, you'll notice that the bitrate displayed is changed every so often. If you close MM and re-open it, the correct values are displayed again.