This type of curve is what I would expect out of machine learning. At first there is rapid improvement as it learns the easy lessons. The rate then slows down as further incremental improvements become less impact.

What is, perhaps, surprising is that human play happens to be relatively close to the asymptote. Although this could be explained by Alphago being the first system to beat humans. If its peek performance were orders of magnitude higher than humans, a weaker program would have already beaten us.