Ground out anything near water or a wet environment (outdoors, even in Texas) or you may get a nasty shock when something goes wrong.
Notice I didn't say if something goes wrong.
Are you measuring the voltages at the switched (controlled) terminal ? If so, there could be corrosion in the contacts inside the switch causing resistance and the voltage drop. If you are measuring at the hot terminal, surface corrosion could be doing the same thing.
Electricity is still a mystery, that is why the schools teach electrical THEORY.