1) Ke Jie has an estimation of Alpha Go's skill. Lee Sedol did not know how strong Alpha Go was. Lee Sedol was very skeptical that a bot could have reached such high level.
2) Ke Jie has been able to study a game where Alpha Go was beaten.
3) Ke Jie has been able to play Alpha Go before, with faster game settings.
5) No one expects Ke Jie to win.
6) AlphaGo has had more time to train. Between the Lee Sedol games and the 60 game series, there seems to have been noticeable changes.
A strong component behind Lee Sedol's performance was that uncertainty, as mentioned in his interviews.
Stronger players actually start complicated fights to prevail through superior reading.
It is possible however to win like this against a stronger player, if you make effective use of time management... e.g: leave lots of aji, then force fights with complicated variations when the opponent has limited time (e.g: towards the end game) to force mistakes or winning by time. These tricks are informally known as "timesujis".
1: http://www.telegraph.co.uk/news/worldnews/asia/china/1219091...
He changed his mind after watching just one more match:
http://english.donga.com/List/3/all/26/527586/1
>But after watching three matches, he said, “AlphaGo was perfect and made no mistake. If the conditions are the same, it is highly likely that I can lose.”
>“As AlphaGo learns endlessly, all human beings could be defeated in the near future,” Ke said on AlphaGo’s capabilities.