For identical weather conditions on the same bike; fast days mean I'm keeping 19 - 20mph on the flat sections and I can tackle the hill to my office at 11 - 12mph because I'll stand up and hammer. Slow days I'll feel like I'm pushing hard and take a look at the computer: 16mph. That final hill to the office means I'm gearing down into my 34/32 combo and spinning up it at 7 - 8mph.

I still can't determine a cause. It would make sense if the slow day was the Monday after I did a 300k on the weekend, or if I was coming down with a cold, or hauling 25 extra pounds of stuff... But that's not the case.