Because benchmarks are really hard to do fairly. There's a great deal of subjectivity in what is a "fair comparison" regardless, since there's always a bit more tuning you can do.

That said, a lot of benchmarks are just completely bogus. It's surprisingly common to see someone claim that they're beating the "state of the art" when they're comparing against a weak baseline. One problem is that people don't understand the systems they're benchmarking sufficiently well to obtain good results.

In this case, I have no doubt the results are much better than they've obtained in Haskell before - I think they've done an impressive job and I don't doubt the value of the work. But if the question is whether rewriting your C code in Haskell would make it go faster... well... I wouldn't bet the house on it.