It's not only about canonicalization. It's also about duplicate content. Any pages that show in the "similar pages" link on page 100 go into the not selected index. At least, that's what I've noticed for my site. As well, it includes all redirects. So if you redirect from www to no-www then it counts as 2 pages. Same thing with any redirect. I'm not exactly sure how much of an effect it has on anything unless you have huge similar pages problems.
If you want to try it (which I don't think you do), simply take off your redirects and the number will go down.
However, I'd be more aware of the "total indexed", especially if it is higher than the total number of pages you have. That number seems the hardest to bring down.