[JacobAziza]

I thought that too, at first.

I posed the question here, and it was explained to me:

P&G is only partially about throttle plate efficiency.

More than that it is about using all available power (engine at full load produces more power per unit fuel than at partial load, and acceleration increases load) and minimizing internal engine friction losses (which is where the majority of the power goes in any internal combustion engine).

To put it in more practical terms: it only takes about 5 seconds for me to accelerate from 45 to 55. During that time I am getting very poor mileage (maybe 10mpg).
But it takes 15 seconds to coast back down to 45 again,during which I am getting infinity mpg (EOC). That gives me an average, over the course of those full 20 seconds of around 40mpg, which is far more than I would get at steady state (which is around 25).

The theory was confirmed when I actually started trying it, and my overall long term average increased 5mpg (from about 25 to 30)

EDIT: BTW, my truck is NA