From what I've read/heard, I agree w/ whats been said too. Usually its either that the mixture of malt and water wasn't fully mixed well, and you have "denser" and "lighter" areas. Temperature of the hydrometer reading does matter, and although at say 100degF it may not seem like much, if its really hot, say 180, its a significant difference. If you know about what the water temps were when you did the OG, it can be adjusted easily. I use my brewing software's tool. BeerAlchemy says, for example that if your OG was 1.052 at 180 deg F, thats actually 1.081 at normal OG temp readings.... I don't have a clue what your reading was, but it can drastically affect the reading if you're pretty hot still w/ the wort.