So it will actually reduce your mileage to obtain better emissions ratings?
The engine controller tries to maintain 14.7:1 AF ratio for all operations except warm-up and Wide Open Throttle (WOT).
Compared to old carbeauted motors, this is a big improvement and you got better mileage while getting cleaner exhaust.
But, with todays technology, you could run motors much leaner and get much better mileage, but that would increase the emissions, so we get poorer mileage to keep cleaner exhaust.
The EGR, by diluting the incoming oxygen with some exhuast gas, will make the motor run leaner in the sense it uses less fuel for the same amount of power going out, but still keep the same apparent AF ratio of 14.7:1, since there is less oxygen. That is why disabling an EGR can result in poorer gas mileage.
BUT, what you folks are seeing is; pulling power from the PCM erases the volatile memory, it does the same as putting the PCM in quick learn mode.
The PCM is adaptive, it does more than adjust to driving style, it adjusts (tunes itself) for production variances in the motor, wear changes, ambient conditions, fuel, etc, etc....
As your old spark plugs were erroding away, the PCM was adjusting itself to compensate. You swap in new plugs, there may be some gain in retuning the motor for the new plugs, and the PCM will do it itself. BUT, pulling the power to the PCM and clearing all the old correction factors the PCM learned and stored to work off, its like putting it in quick learn mode and it adjustments the PCM makes have a greater effect faster.
If there has been no change to the motor, likely the PCM has learned all the best settings, and clearing the volatile memory would have the opposite effect, it would make the motor perform worse and get worse mileage until it has tuned itself/adjusted back to the correction factors prevously stored in it.
If people are getting improvements clearing volatile memory in vehicles that have had NO changes to the motor (and I have seen posts for other vehicles with people claiming to see improvements) its a fluke, or more likely a temporary condition where the motor is likely making more pollutions and with that better mileage/performance and will quickly tune its way back to what its setting where before.