2021 Tahoe high country fuel tank capacity discrepancy

[Lsxjay]

My 2021 Tahoe is supposed to have a 24 gallon fuel tank. If I go fill it up when the low fuel light comes on, it will only take 16 gallons. If I wait until it says low, it takes 17.5 gallons.It got irritating to the point that I purposely drove it until it ran out of fuel. I had a 1 gallon container in the back. Put the gallon in it and went straight to a gas station. Filled it up and it took 19.1 gallons. So either it only has a 20 gallon tank or it has 4 gallons of unusable fuel. I had the dealership check the sender and nothing was wrong. They don’t know what to tell me. Has anybody else run into this?