It is purely by score (with wins capped at 14 points so blowouts don't skew the calculation). The reason you may see movement during the postseason is related to the way the rest of the power rankings migrate based on the post season games, i.e. every team plays teams who are in the playoffs, and so during the playoffs those teams will get adjusted, which in turn impacts the other teams, and of course this recursively effects everyone until the algorithm resolves.
I think I posted this in another thread but the algorithm is actually very simple (you can do the same in a spreadsheet using a solver tool) - it actually came from this book:
http://tinyurl.com/hjxaqrz
