Quite possible I don't fully understand how these things work.
In my case, since the car was new (4 miles on the odometer) up until now (9 years later and 150,000+ miles), the manual mpg pretty much stayed the same - about 22.5 mpg.
To me that means the sensors, the computer control, injectors, etc etc have not shifted/changed much.
For the first 110,000 miles or so, both indicated mpg and manual mpg matched . . . meaning the K-factor setting done at the factory was pretty close. Then the indicated mpg started to drift down, while the manual mpg remained constant.
If the electronics/sensors used to compute the mpg is different from those used to operate the vehicle, then I can understand your explanation - the electronics/sensors used to operate the vehicle remained constant but one or more sensors unique to the indicated mpg electronics started to drift and is now causing this difference.
I find it hard to believe there is a completely different set of electronics/sensors to compute the indicated mpg.
I suppose best to stick with manual mpg . . . no ambiguity about that figure!