From what I can see, for the puzzles that have records, the highest score on the graph is 5 times the median. The other two numbers on the graph are just the mean of the median and the quickest record and the mean of the median and the 'slowest' score, which is technically just 3 times the median. I presume, as alamakota said above, that the median score is an actual true sample, as is the quickest score. I think the bell curve is just there as a visual aid since it never changes. Even if all of the puzzle solving times did wind up as a normal bell curve, you'd expect some variation between puzzles if you were graphically representing the distribution of solving times.
