2021 Blazer Fuel Capacity

Do you always wait until the fuel light has been on steady for a few miles, then blinking for more miles before getting to the fuel station on the fumes?

Most of us know that can lead to issues by drawing the dregs from the bottom of the tank into the fuel lines.

I would expect to fill up while there was still at least two gallons in the tank, so should not need the full tank capacity.

Pumping extra in takes up the expansion space and can lead to blown fuel tanks. This happened in the past when a manufacturer built tanks with the breather pipe in the wrong location and had to recall vehicles to swap out the tanks.

Most of the automotive fuel tanks have in-line fuel filters, mechanical floats for fuel level measurements and mechanical fuel gauges..

The fuel filter has a builtin cartridge that may already be soaked and partially clogged.. Coupled with the type of the mechanical float used inside the gas tank together with the sluggishness or responsiveness of the mechanical metering used in your fuel gauge when combined with variations in ambient temperatures and the eye parallax may be the reason that causes the differences in your observations...

How do you know how much fuel you have in your tank?

Good point; unless the OP has a very accurate gas gauge or sits at the pump until he runs out of gas, an accurate comparison seems impossible.

This explains how the gas pump shutoff works and how it can be defeated.

https://www.youtube.com/watch?v=FRYlTF02A7g

On my 2000 Ford F-150, if I turn the nozzle upside-down when filling, I found it will overfill the tank. I discovered this once when I was filling up with the hose stretched across the bed. I suspect it have something to do with the location of the vapor pressure sensing port relative to the fuel flow.

Fuel tanks have a shut-off valve which limits over fill which is necessary to allow for expansion in hot weather. Forcing more gas in the tank can lead to a saturated Carbon Canister which can lead to emission problems and the danger of fuel leaks due to internal pressure in the tank.

One piece of missing info in your post. You say that you can only pump 15.5 to 16 galls. before the pump shuts off but you don't say at which point you refuel.
Is your fuel gauge reading empty, is the low fuel light on or what is the gauge reading?