Back in the day, I worked at a gas station. Every so often, we would have a customer who would come in and top off his tank a bit at a time for almost 20 minutes. (This was before vapor recovery nozzles.)
The 20 minutes yielded maybe an extra gallon.Some people are obsessed with topping off their tanks.
I would have to agree with some of the comments that you are overfilling your tank, causing fuel to go where it does not belong, such as the vapor canister and the fuel is vaporizing from there.
For the life of me, I don't know how you can tell its missing, or why you should even care, unless there is evidence of a leak. 3/4 of a gallon of gas, spilled on the floor is a significant puddle.
Best of luck.
