My best guess is that they're determining percentiles relative to other test takers, and that people who spend time taking IQ tests online are unrepresentatively high IQ.

I think this is likely; I seem to recall iqtest.dk saying something to that effect. Given the various reporting biases involved, though, I'm unwilling to jump immediately to that as a conclusion. I recall the Raven's numbers being lower than what you would expect given the SAT numbers, but being closer to the SAT numbers than the self-reported IQ numbers, which were higher than you would expect from the SAT numbers.

That is, even if I agree with your prior that LWers do better on Raven's than on other tests, observing LWers doing worse on a Raven's test than other tests should reduce my confidence in that, rather than me just using the prior to adjust the evidence to agree with it. (Administering a properly normed test, of course, would screen off the improperly normed test.)