MOre MPG values when engine spinning?

Something interesting I've noticed:

When the car is warmed up and going less than 42-45, the instantaneous MPG is nearly binary: it's either pegged up at 100 MPG when I'm coasting/gliding, or it's in the range 10-35 MPG (depending on my acceleration), nothing in-between.

But when I'm going above that speed, say 65, it can take on any value from 10-100 MPG. It's not unusual for me to be able to sustain an indicated 75 MPG for quite some time on a level or slightly-downhill run at 60-65 MPH.

The speed cutoff leads me to believe that this has something to do with the engine spinning, so my question is what's happening? If the engine is spinning, can the computer be cycling firing on and off so that I'm getting a half-duty-cycle of sorts and sustaining 75 MPG?

It doesn't seem like it's an EV-boost kind of thing, as that should be able to happen at lower speeds. (In fact, I'd assume it would be more likely to make a real difference at lower speeds.)

Ideas? Insights?