Well in my mind a single throw switch is basically on or off,a double throw could be on or off either direction like a 3 way light switch.
I guess Ive just never thought of it in terms of a thermostat for a water heater
"facts" have no relevance to this discusion
Posting from a concrete bunker under a non descript barn
are are referring to the difference between the upper and lower thermostats? the upper one switches the power to the lower thermostat after the upper part of the tank reaches temperature. on a cold tank the upper heats first and when the thermostat shuts off the upper element the lower thermostat is energized.