This is misleading. AlphaGo beat a 2p player five months ago. Now it has beaten a 9p player. That tells is nothing about it's improvement in the intervening time. Given only this information, however unlikely, AlphaGo could have actually been stronger before.
Replying to scarmig, No. Player A may consistently beat Player B who may consistently beat Player C who consistently beats Player A.
There are different strategies depending upon how much emphasis is placed upon early territorial gains as opposed to "influence" which is used for later later territorial gains.
Similarly, playing "passive" moves that make territory without starting a "fight" versus agressively contesting for every piece of territory available.
It's more that there are different styles of winning, and a human will tend to specialize in such a style. But generally, someone who is consistently stronger will win over someone who is consistently weaker, regardless of style.
