Is it normal for my hardware hashrate to vary by 15% but still achieve the advertised average?
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.