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They count the replys to the answers as well. So if someone replys to an answer 20min ago it'll say that on the question page.
Answers that get helpful votes etc can change the order of comments. *shrug*
This happens with the William tyrell post for me. Keeps coming up new but no one has added anything to it for months!
This has happened a few times to me. I think they just remove a comment and it still shows up as "last answered ... Ago".
It could possibly be that we've removed a comment, or it may just be a glitch (which can happen sometimes). We'll look into it and see if we can find out for you :-)