The theory (and it isn’t mine, but I tend to agree with it) goes like this: the power required to overcome aerodynamic drag is cubic to speed, meaning that the additional power (watts) required to go from 55 kph to 57 kph is significantly greater than it is to go from 53 kph to 55 kph, though both are increases of 2 kph. Simplified heavily, that means that if the breakaway can go a bit faster than before, then the chase has to go even faster than that, and the power increase required to hit the new speed is greater than the power increase for the break to hit its new speed. At some point, you run up against what’s possible on tired legs, two and a half weeks into a Grand Tour.