Just to add a few notes in addition to Pieter's answer, and with a hint to your implied question (can AlphaGo beat Lee Sedol?):
First off, I'm not sure if you thought being 9p automatically means being stronger than 2p. This is not the case.
However, in this particular instance, other matters are of more importance. Fan Hui has not been competing in Asian (i.e. highest level) tournaments for many years, and his competition in Europe is limited - that's why he's the current European champion, after all.
However, this also means that it can be assumed that he's not quite as sharp as an active, competing pro in Asia. His focus simply shifted towards teaching, as teaching was the reason he came to Europe. That is to say, and with no disrespect to Fan Hui's great skill, Lee Sedol, an active top level competitive pro, is quite a bit stronger than Fan Hui - 2 stones, goes the rumor among pros.
Obviously, the closer we get to perfect moves, the more difficult it becomes to improve further: For instance, doubling AlphaGo's calculation power does not at all lead to doubled strength. Think of it more like a logarithmic scale: To improve another stone in strength, you have to put in just as much work as you did for all previous stones together (roughly speaking). It remains to be seen if AlphaGo's improvements since the Fan Hui games are large enough.
About comparing pro Go players
While we cannot compare pros by their rank, we can get a rough idea of their relative strength by using Dr. Taeil Bai's chart, of which I reproduce the beginning of the most recent version I could find (from cyberoro, 2014):
I am not aware if Fan Hui is included in the list, too - if he was, that might be the source for your 600th-700th information.