Water heater thermostats only control (switch) one leg. The leg that is not switched feeds voltage to and through the element when the thermostat is open. So you will read 120 volts to ground on each end of the element and zero if reading across the element. (The thermostat will read 240 volts across it when open.) When the thermostat is closed, you will read 240 volts across the element and zero across the thermostat.