Fuzzers have some inherent nondeterminism, meaning that some runs agains the same program will differ in how many bugs they find.
It turns out pretty much every fuzzer evaluation to date has not been statistically rigorous. We dont actually know if a lot of published results are, well, really an improvement.
There is also a fascinating bit towards the end. It turns out doing the dumb thing and not tracking code coverage finds more crashes faster than intelligent coverage tracking, but only with good seed inputs (see page 7).