It seems as though they might be fitting to a lognormal distribution rather than normal distribution?
I would love to see the raw data of solution times.
Normal distribution can't be the case since it would allow for solution times less than zero.
Just did a quick analysis for one puzzle where the median solution time was 194 and the z=1.5 time was 112 and the z=+1.5 time was 582 .... 1.5sigma either 82 on low side or 388 on high side.
If you take log of times, then the z=1.5 log time is 4.7185 and z=+1.5 log time is 6.36647 giving a lognormal 1.5sigma estimate of 0.549 on low side and 1.099 on high side ... closer to equitable but still different enough to make me wonder what the underlying distribution is.
