My question: My Antminer control panel has been consistently showing 502 MH/s with very little variation. But, on my Litecoinpool Account page, the hashrate can vary +/- 50 MH/s.
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Do you think that it is because Litecoinpool is using some calculation to guess what my miner's hashrate is, instead of knowing what it is?
Yes, that's exactly it. Miners do not communicate their hash rate to pools, so pools can only estimate it based on how frequently shares are submitted. You can find a little more information in
this FAQ.