Cadillac Fuel Gauge Discrepancy

For every tank or so of gas I use, the "fuel used" indicator on my 1990 Cadillac Fleetwood (Front wheel drive, 4.5 liter V8 ) says I've used somewhere between 1 and 2 gallons less than what fuel I've actually used (takes more at the pump). This has been the case for a couple of years, and I've tried many things and have been unable to phase this problem.

The following things have been replaced at some point during this time, and can be eliminated as they have had no effect on the problem:

Fuel tank and sender unit, fuel filter, fuel pressure regulator, distributor cap, rotor, plugs & wires, alternator, air filter, oxygen sensor.

I also have a receipt from the person I bought the car from which shows the catalytic converter was replaced, so that is doubtful. I have also placed bosch injectors in the car. They were from a salvage yard, but are known to be a superior plug to the stock. In any case, the discrepancy remained unchanged, so even if they aren't perfect, the stability of the problem tends to indicate it as not the problem. I have done a fuel pressure test, and it has acted as it's supposed to with either set of injectors. Except that it does slowly bleed down on key off or after the fuel pump is finished with it's 2-second priming when the key is in just the "ON" position before. However, I did try clear flood mode (holding the pedal to the floor before starting and supposedly shutting the fuel off), and it starts every time that way, when it is not supposed to. It will do so immediately though, before any pressure has time to bleed down, so I'm confused as to how fuel could be getting in there other than an overall rich mixture.

There is no visible fuel leak in the lines along the way. And certainly no leak that could account for the sheer amount of fuel in the discrepancy.

There are no diagnostic trouble codes. The oxygen sensor has cross counts in closed loop as it should. The radiator and thermostat have also been replaced, just for reference, though I don't see what they could have to do with it. Except that they could have an effect on actual mileage.

The actual mileage is not horrible, but is not what it should be. It is rated for 25 on the highway, and I'm getting maybe 22. If I were getting what the fuel data center thought I was, it would be about right.?

I know these fuel data centers are capable of being more accurate than this. Maybe not on the fuel left in the tank, but on the fuel used, they are. I had an 85 which was a mechanical disaster compared to the 90, but had the same fuel data center and was never off by more than 0.2 gallons. So anyhoo, I'm at a loss for what causes the discrepancy. Any help or ideas would be greatly appreciated.