i think the center point has a lot more connection that the sides, so there are a lot more variations, so much more that it is very hard for alphago to handle, especially that the edge were sort of established, which created a situation which is (1) total new to the AI (2) complicated enough that any meaningful search is hard to be exhaustive.

--- this would explain why alphago miscalculated between 78-87. to explain 93, 97,131 etc, it is probably something 1fD7 pointed out.