I do think it is related to fuel consumption but whatever the reason, at times on cross-country trips it can be maddening, if you like to drive at night as I do they can take over the road.
One gets used to keeping an eye on the trucks and anticipating when they are going to do it which means we car drivers hit the gas and bypass both of them before it starts.
I know some of these 18-wheelers are governed (like SWIFT trucks) so they won’t go faster than, say, 67MPH so I suspect the problem is you get one governed truck trying to pass another governed truck at a 0.01 MPH speed differential.