jarlrmai
Veteran
I think it is actually quite easy to misjudge relative speeds.
When you are gradually creeping up on someone, your closing speed is helped by (a) the psychological effect of seeing them ahead of you and (b) however modestly, by aerodynamics. So you think to yourself that your closing speed justifies overtaking. But as soon as you overtake, both those factors stop working for you and start working often person you've overtaken. And you find yourself not pulling away as fast as you expected - or, from their perspective, being a nobber
Happened to me on a 2.5 mile 5% avg the other day misjudged the upcoming gradient and went past someone then was unable to maintain that pace on the road but they were able to up thier effort.