You're assuming those are random numbers. As the article says, they are only semi-random (that's one of the big surprises they found).

And you can see from the numbers that it's not just Mitchell's values that deviate from the expected mean, it's the others as well (he's just more extreme), which also shows we can't expect them all to converge to the mean, there's some more complex statistical process going on.

His deviations being more extreme might be explained by him playing more games, or it might be cheating. So far we can't tell.