Is it normal for my hardware hashrate to vary by 15% but still achieve the advertised average? I have an Antminer S1 and an Antminer C1. They both work as advertised and achieve a three hour hashrate average of 200 Ghash and 1000 Ghash, respectively. However, within any given second they hash within 15% of that average. So the C1 right now is hashing at about 850 Ghash, but I've seen it has high has 1150 Ghash.

I thought perhaps that the S1 did it because it is overclocked. But the C1 is not overclocked. I also think perhaps that my source for the stats might be the problem, which is the Eligius stats page for my miner.

Is this kind of fluctuation normal? Why does it fluctuate like this? Is this specific to Bitmain hardware or does other hardware do it too? If it is not normal, what could be the problem?
I have TechnoBit Hex miners running BlackArrow ASIC chips and my equipment fluctuates a lot. I mine with the 'Discus Fish' Pool (aka F2Pool) and they let me view my hashrate on their website on a really fancy graph. My 1200 GHs dips from 900 GHs up to 1300 (occasionally), but in the end I do get a nice even 1150 / 1200 GHs.

The software (Cgminer) does fluctuate even more on my screen, it goes from 700 to 1300 GHs, but this really depends on what difficulty your miners are getting from your pool. Anyhow, a little fluctuation is perfectly normal.