The "Not Selected" isn't cumulative. The "Ever Crawled" is though.
I have a large Wordpress content site. It was hit by Panda on a very same day that my "not selected" multiplied by 8. I don't think it was a coincidence, and I didn't make any large changes to the site besides the regular addition of about 10 posts per week.
I've been able to effect a downward movement on the not selected count by removing/redirecting things like "replytocom" variable URLs in the comments section;reworking print and email versions of each article, etc. It very slow though, only reducing by an average of 100 per week.
Needless to say, I think the not selected metric means quite alot.