When you have the exhaust/muffler below the engine, fuel overflow when filling the tank will run out the muffler without the posibility of flowing back into the cylinder and causing a hydrostatic lock. With the engine in the upright position and the exhaust straight back or (as most mount it) at an angle up, the fuel can flow back into the cylinder and most folks will pull off the pressure line each time they fuel up to let the fuel overflow safely.

quint