The sitemap is simply an extra "hint" for Google as to what you want indexed. It's not a definitive list. It will still crawl your site for pages and it will still update pages that are already in your index. If it indexed a thread that you since deleted, it will still hit it (and it would discover a 404/410 and eventually remove it from the index).
Worth noting that the particular example shows Google last hit the page on the 10th. It says it was in a sitemap, but it's not necessarily in the latest version -- they'll have the same URL so you can't necessarily know which "version" it'd be in. But if it hasn't crawled since the 10th, it's a pretty good indication that it is only stating that the URL existing in the sitemap as of the 10th. (Clearly that's not 100% foolproof but then we're talking about internal Google machinations that we can't really know.)
Finally, note mostly that the webmaster tools output is mostly designed to be informational. If you find a sudden jump in 404s or other errors and you didn't make a change/expect that, then it's worth looking into. But if you made changes intentionally (say you removed all of the threads in a forum), then there isn't really anything for you to do.