CBR 192 encoded MP3's will be capped at 192kbps, no more and no less. Most tracks have quiet and busy moments. Quiet moments don't need 192kbps, so with VBR and ABR the kbps will drop to a really low value. That means that this leftoever bandwidth can be used for more complex passages in the song which otherwise 192kbps would not suffice.

ABR will be the same size, whereas VBR can differ wildy. With ABR it is easier to determine space requirements, whereas with VBR the emphasis is on quality. An mp3 encoded using ABR will sound better than an equivalent encoded CBR file. This is only of any use if encoding less than 320kbps. The ultimate quality (warts and all) would be CBR 320kbps.