Actually, according to Probst, if the degradation occurs gradually (as with most components that get more inefficient over time), the computer can accomodate quite a lot of change. However, if you can unplug O2's and the maf with no check engine light - well, something is clearly wrong. You should count your blessings that whatever has gone wrong has occurred in a way where your emissions and mileage have improved - as you know, that's rarely the case.
I'll trust your judgement's on your fillups and calc. procedure. But know that simply filling the tube until you get some overflow doesn't assure that the tank is completely full up to the top of the neck. The way modern tank necks/tanks are designed/vented, you may have to fill to overflow, and then wait 30 seconds to a minute, and fill again multiple times to actually physically fill the tank all the way to the top. The good news is that keeping track of the mileage over a long period of time will average out the highs and lows and give you an accurate idea of your mileage.
All I can add is that if you're actually accidentally getting that mileage with a computer/systems that are malfunctioning, you're probably running very lean. I'd watch it carefully for lean/detonation/heat related symptoms.
