My own are pretty consistent over the same range.

You haven't mentioned another possibility: that it might be network congestion somewhere, perhaps restricted to your ISP or even your locality.  Even though I'm on cable broadband, my own reported hash rates tend to slow down in the evening (from around 17h00) as everyone gets home and turns their computers on, which sometimes has a small intermittent impact on my mining for five or six hours - typically up to 10%, which is about what you are seeing.

I also see a significant difference if I switch from cable to ADSL (different ISP), when I get as much as a 25% reduction.