Ask HN: Why are some very old submissions appearing in the first few pages?

I was browsing beyond the first page of HN today and noticed something very unusual:

http://imgur.com/VqVXX

How is it possible (not having any real knowledge of the weighting and ranking algorithms used) that some extremely old submissions have managed their way to such recent pages?

I would have expected that age of submission would be a major weighting factor in determining story rank, so perhaps there's some weird bug in the algorithm?