Not sure how helpful / accurate this is but I had an phone interview with an OSU professor a couple weeks back. When I asked about funding at OSU he said that typically get X dollars from the University for PhDs and then depending on the class they'd like to admit, they attempt to scrounge up Y more dollars from the Poli Sci Department. Because of that somewhat haphazard process (this may just be standard procedure for most universities, I have no idea) it takes them a little bit after acceptances to know what the funding situation is going to be. I have no idea if this interpretation is correct, but depending on what Y ends up being, they may be able to fund more (or less I guess) students than they originally expected.