The spikes are from hard problem instances - he's not running on the same data set each time. From Norvig's post: But the main message is that the mean and

The spikes are from hard problem instances - he's not running on the same data set each time. From Norvig's post: But the main message is that the mean and median stay about the same even as we sample more, but the maximum keeps going up--dramatically. The standard deviation edges up too, but mostly because of the very few very long times that are way out beyond the 99th percentile. This is a heavy-tailed distribution, not a normal one.