If we were to assume a test with a standard deviation of 24, a median of 141.5 would be just below the 96th percentile. That still seems too high for the median user, but it's almost plausible -- much more so than 99.7th percentile.

It's also quite likely that LW readers with abnormally high IQs (relative to LW) are (A) much more likely to have been tested and to know (and remember) the result, and (B) include the score on the survey.