One thing that's always surprised me, but probably shouldn't, is how quickly your moving average can come down compared to your steady-state pace. For example, when I'm riding at what seems to me to be a consistent 16-18mph, I'm lucky at the end if my overall average is even 14, and on a multi-day road tour I am closer to 12mph even though I feel faster. The issue, of course, is that the slowing down or stopping at road crossings or tight turns and then getting back up to speed takes more out of your average than you'd think. And when you slow down on a hill, you do not really make it up on the backside.

Also, it's a lot easier to drop down and cruise at 6mph less than your fast pace than it is to ride 6mph faster to make up for it. As a result, once your average drops off a bit on a long ride, it's very difficult to pull it back up. (I play these games when I ride with my Garmin with all the readouts.)