Sender float is sticking/stuck.
Typical and common issue with older vehicles which sit for long periods of time (I had a 2003 F250 that had a lot of miles on it and eventually had the same issue with the sender sticking when tank was low and you filled it full and the gauge didn't go up for many miles).
Options are:
1) Live with it.
2) Use a 2x6 cut to a ft or so long then use a 3 lb hammer to "tap" on the tank sides and bottom (hit the broad side of the board please).
3) DROP the tank to remove the fuel pump/float assy and REPLACE..
I would attempt #2 (kind of what I did and it seemed to work for a while), then if no joy, #3 would be the best way to fix correctly.