Actually VBR is shown to give higher sound quality, and at lower file size. Someone posted a really good link recently that I'm trying to find, with graphs comparing all kinds of different encodings. From doing my own listening tests, I had found that I was consistently picking the high bitrate VBR over the CBR, and when I read that article it really sealed it for me. Don't always assume that more bits necessarily = better quality.
But for 320kbps it can not go any higher, just lower. Hence CBR always have the maximum bits, while VBR may not.
But, what do I know...