It's because the pools estimate your hash rate based on submitted shares, while your cgminer has access to the actual hash rate. So if you're lucky and find a lot of shares quickly with your constant hash rate, the pool will think you are hashing faster, while if you're a bit unlucky and don't find as many shares for a while, it will think you're hashing slower. As you said, it all evens out over time. If it doesn't, maybe you're creating a lot of invalid/stale shares.