Everyone knows what their "best average" is - on a sunny, windless, perfect day in top fitness - but how often does that come about? I've only once averaged 19.5mph on my commute, in both directions, the rest of the time it's 17-18. I could keep up at least 20 all day if it was relatively flat and good weather, I don't tire or need a break until after 3-4 hours.
So if they really do "average 20" then what's their "best average"? There are some v v fit people around of course - the ones who got round the Elenith in 12 hours last year, for example. But I ride a 'bent and I've never been overtaken by ANYONE on the flat, and as I say 17-18 is my longterm average, so I just haven't met any of them yet.
And the track standards are pathetic, these are the people who'll strap their bikes to their car and drive home, totally irrelevant for 'real' cycling.