Not everybody remembered to stick FM before their title, so there are a few posts that aren't going to come up if you search that way.
Also, I sort it by month, not year. If you sort by year, you might have to look pretty far down in the archive to find even a more recent post if it wasn't zapped a lot.
Don't know if I'd call it a bug, but SN's search function is notoriously odd. I'm not quire sure how it works when you sort by zaprank while using a keyword, but I bet that's where the difference comes in.
FMbefore their title, so there are a few posts that aren't going to come up if you search that way.