Can't speak to generator issue, although your point about the dip tube not extending as far as dip tubes for the mains could be on target. I've read that's not uncommon.
But as to 390 gallons... gauges, especially some gauges, may or may not be in the same ball park, let alone be accurate. And some systems intentionally build in a reserve. And then if they're analog gauges, judging what it means when Mickey's hand is on the E may take running out of fuel once, just to see.
Also, if your fuel senders are adjustable (some are), they may be out of adjustment.
And... the shape of the tank impacts how the sender works. For example, in a cubical tank, the sender float could well be at a different point (sending a different resistance in Ohms) than it is in your tank... even with the same volume of fuel in each tank. (There are some gauges that can sort-of deal with that, and which have some calibration capability to adjust for tank shapes.)