I always wondered why the "Safe Fill Level" was significantly below the top of the fuel tank
. . . . turns out that gasoline does expand as a liquid as it warms. The vapor pressure increases, too, and most people think that's all you need to worry about. But the expanding liquid will exert much more force if not given enough "padding" above it. If you filled the tank clear to the top and closed the cap tight and left it in the sun, it could burst the seams of the tank.
So, long story short . . . don't worry about expansion if the fuel
level is below the safe fill level.