It's quite simple. Effective hashrate = NumShares x ShareDiff / Time ... that's what the pool does. It may add the rejected ones too when displaying hashrate (I don't know for Flypool; Ethermine does not).

Looking at the the bminer1 log, you had 4880 accepted shares, 16000 difficulty, and 27360 total seconds (give or take). That gives 2853 h/s. However, this is the total average over 7.5 hours. Pools show you averages over smaller windows, e.g. 10 minutes, and that will vary a lot more, especially if you have a high share difficulty.

Note that the above hides the miner fee. Bminer does not report the number of shares it computed for the fee when it reports the number of accepted shares, but does include it when it reports the hashrate (I know, it's pretty twisted). Happy to be corrected by @realbminer as I can't know for sure, but the computed hashrate from the shares reported by bminer does not match the hashrate.