> That’s over ten times as many games as estimated.
That's still a pretty good estimate of an exponentially large quantity; the exponent being off by only 1. With these estimates you cannot hope to do better than estimating the exponent.
But for chess, the spread in number of moves is a lot larger, and the branching factor is higher. 20 more half moves and a branching factor of 35 isn’t unreasonable, and gives you an underestimation of over 10³⁰.
