This explains a lot. Going into the match, Sedol thought he could beat DeepMind but thought it might only be a couple of years until the technology outpaced him. We knew Fan Hui and other Go professionals were helping the team, but a massive speedup is always nice too.

It's a bit underhanded, however. IMHO, the player should be able to study recent games before the match. But this is pretty typical, there were similar late-stage improvements with Chinook (checkers) and Deep Blue (chess).