Alot has to do with mpg's. Weather, terrain, 4x4 vs 4x2, Lifted vs not lifted, A/T tires vs street tires. The way you stop and start,......
........elevation/air density, where you bought the fuel, how much moisture and contaminants are in that particular holding tank, how long the fuel has been in the tank/if it is a fresh batch that was just delivered, etc, the ethanol content is a huge performance killer, both in BTUs and mileage and there's the difference between summer and winter blended fuel.
All else being equal, I dropped from 12-14 mpg city, to just over 10, simply from the switch to winter blend.
Other than that, all the other factors above, contribute to fluctuations across the board. My best mileage is in summer, when I use non ethanol fuel and drive like a gramma.
I'm still not convinced of this "learning" curve that I hear the ECU has. I think it's simply a matter of all the factors listed above. Which, vary from gas station to gas station and day to day. So, I guess I'm saying that it is all dependent on all these factors and your truck itself won't make any decisions to change, nor will it be much more broken in than it is right now. Except maybe parts wearing out. But that would hurt mileage, not help.