There seems to exist a lot of false positives; for instance, I included Lisp on the chart, and it indicated 7 job posts in May '13. I went to the whoishiring's posts for that month (both the "Who is hiring" [1] and "freelancer?" [2]) and I counted 6, but only one of them (the first instance, in a "top level" comment) is actually a job posting (all the others occur in sub-discussions). And as it turns out, it seems that it's only a sort of "eye-candy" (although I admit that filtering these may be harder).
My point is, you might be able to avoid false positives if you only consider "top-level" comments.