The performance of doctors follows the normal distribution. I think that extrapolating to programmers is not a huge stretch.
It used to be assumed that differences among hospitals
or doctors in a particular specialty were generally
insignificant. If you plotted a graph showing the
results of all the centers treating cystic fibrosis—or
any other disease, for that matter—people expected that
the curve would look something like a shark fin, with
most places clustered around the very best outcomes.
But the evidence has begun to indicate otherwise. What
you tend to find is a bell curve: a handful of team
with disturbingly poor outcomes for their patients, a
handful with remarkably good results, and a great
undistinguished middle.