AlphaGo maximises the probability of winning, and not the margin by which it does. So those "mistakes" yesterday turned out to be fortifying moves because AlphaGo was confident of a win. And similarly today the weird moves were interesting because they perhaps indicated that AlphaGo thought it was ahead.
Yeah, that explanation from the DeepMind team member today put a whole new spin on some of the 'odd' late game moves. It doesn't 'care' about about margins so it will shore up its odds of a win in preference to increasing the margin if it wins.
I had a feeling after yesterday's win that people might be tempted to call it a close win by AlphaGo and predict that Lee could overtake it with a bit better play, but that we'd find no matter how good Lee played, AlphaGo would adjust and continue to come out just a bit more on top. It's actually probably really hard to tell how much better AlphaGo is because it probably plays quite conservatively overall and hides a lot of potential strength.
