> It sounds like the opinion from the expert is that there are some inherent weaknesses in how MCTS tends to play moves against theoretical moves from the opponent that don't make sense; ie. that AlphaGo sees a potential win that would only happen if the human player made very bad moves.
Why can these very bad moves not been pruned from the search?
Some can, but as you can see with how human players commented on the early AlphaGo moves, you can't necessarily objectively quantify moves as "good" or "bad" correctly without exhaustive search, so you make an approximate prediction and go from there.
But for every threshold of calculating that, you'll always either see moves that are just "good" enough to not be below the threshold (and get "why can't we prune these out"), or just "bad" enough to require it explore that space of the tree if a human player unexpectedly chooses them (e.g. the brilliant move that came in game 4, and AlphaGo's figurative loss of equilibrium.)
Why can these very bad moves not been pruned from the search?