Sorry for thinking it was a kind of p2pool

I am using the France server (30ms), mining at 313MH/s and the pool says I have 238-242MH/s
how many rejects does the miner show? can you please give the statistic from miner? thank you!
Same here, I'm mining at ~2000 and pool says ~1600.
I have reject rates between 0.6% and 1.4% - that doesn't explain a 20% underreporting. What gives? Is this because of no-submit-stale? (i.e. is stale shares displayed in the cgminer hashrate?)
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GPU 0: 73.0C 50% | 221.8M/221.8Mh/s | R:0.6% HW:0 WU: 50.3/m T:1 I:14
GPU 1: 74.0C 1862RPM | 145.0M/145.1Mh/s | R:0.6% HW:0 WU: 31.3/m T:1 I:14
GPU 2: 73.0C 19% | 303.5M/303.5Mh/s | R:0.5% HW:0 WU: 66.8/m T:1 I:14
GPU 3: 74.0C 2640RPM | 198.1M/198.1Mh/s | R:1.4% HW:0 WU: 44.7/m T:1 I:14
GPU 4: 74.0C 2048RPM | 211.2M/211.2Mh/s | R:0.6% HW:0 WU: 46.1/m T:1 I:14
GPU 5: 74.0C 2844RPM | 303.5M/303.5Mh/s | R:1.0% HW:0 WU: 68.4/m T:1 I:14
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GPU 0: 73.0C 3647RPM | 456.7M/457.7Mh/s | R:0.8% HW:0 WU:101.9/m T:1 I:14
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[2014-02-14 00:44:15] accepted: 7918/7970 (99.35%), 66103 khash/s (yay!!!)
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GPU 0: 73.0C 3081RPM | 131.8M/132.0Mh/s | R:0.8% HW:0 WU:27.7/m T:1 I:14
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The displayed hashrate is only rough approximate. The formula is based on the count of your shares and some constants. payments do not depend on it! your payouts are calculated on the real sent shares.
I have tuned this formula, now you should see nearest hashrate.