I would be surprised if it made that much difference, unless they were just half flat the first time. Anyway, if you're comparing speeds, it's good practice to check them each time just to eliminate a variable (and to make sure they're not half flat).

Riding the same course the same way on two different days, my times or speeds are seldom the same. One thing I notice is the wind, or lack thereof. Another is the heat, and I'll do noticeably better when it's more pleasant- I can work harder, and don't have to carry as much water.