No biggie, but I've certainly noticed that as I have big wins drop off the last 120 days my ranking dips and if I have a 120 day period without much in the way of decent finishes it gets worse still.
Also if you look at the rankings under the OPR tab the summary stats that are shown for people are those for the last 120 days. There is a pretty good correlation between these and the ranking.
The prize rankings and profit rankings are also definately based on the last 120 days.
It may not be the only factor but it seems to be at least part of it.
Can't find anything via the rest of the Intraweb, this was the best I got.
Review of Official Poker Rankings