Its all about terrain. I usually average 12mph but can easily cover 1800ft of climbing in 20 miles whereas the 18-20mph folk, 9 out of 10 times are on flat bike paths or live in Texas
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I was amused a few months back when a member who regularly posts 18+mph average posted his garmin stats He 'climbed' 300ft over 15 miles and complained it was a 'rolling hills' ride. I have to climb more than that leaving the driveway.
In March, we went camping in another state and I took a ride around the area - 50 miles later (and less than 800ft of climbing) I was fresh as a daisy. Its amazing how far you can go when things are flat.