My miner show constant hashrate about 1kH/s, but on miner statistics average hashrate is 20% lower:

Current Hash Rate: 883.00 H/sec
 Average 1 Hour Hash Rate: 832.80 H/s
 Average 6 Hour Hash Rate: 855.33 H/s
 Average 24 Hour Hash Rate: 814.48 H/s

Why?