sometimes yeast are over performers... also, other things in your wort could help the yeast eat more... Things like pH etc could, but I'm not sure. Mashing slightly higher, increases the kinds of sugars that don't get digested, so you'd get less attenuation, etc. So if you mashed at, say 157 it may have stopped a bit earlier. There are TONS of variables in that mix, though, water could be one too. Yeast are living things and don't always behave like a machine.