The main reason is gas gauges are not accurate. After all there
is no real reason for them to be. As for the "miles per tank",
that means very little. If you want to find out your mileage,
fill up the tank full and record the miles. Now every time you
fill up, always fill it full and record the miles and gallons
to fill up. After a couple of fill ups, you can do the math of
miles driven to gallons used.
My overall mileage on MY99 2.5RS is 23.4 MPG. What I find funny
is that my wife's LL Bean H6 has been getting 25.2 MPG.