Well you are making the assumption that the gas meters on several different gas pumps are all going to be exactly the same which they won't be. So that's another source of error.
Regarding tank size, most manufacturers assign a reserve at "empty" of a few gallons so when the tank reads empty you still have a little time to get gas.
Gas tank gauges are always fairly innacurate I wouldn't lose any sleep over it. Don't forget the gas is moving around in the tank too so the gauge can't be too sensitive otherwise it would be moving all over the place as you drive.
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