What do line voltage thermostats have that low voltage thermostats don't?

Line voltage sometimes varies according to how much load there is from neighbours and other users. When the voltage is high, the water heats more quickly so that the thermostat cuts out sooner, so the amount of energy used ia about the same.

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It is the difference in construction of the two thermostats. A line thermostat uses a bi-metal sensor to open and close the line voltage. The room temperature fluctuation has to be greater between the stat turning on to off, as the differential setpoint can not be regulated as fine as a low voltage stat.

A low voltage thermostat uses a Mercury switch that is operated from a spring that senses the room temperature. Low voltage thermostats do not control the line voltage to the heater but use a remote contactor to open and close the voltage supply to the heater. There is a compensator in the low voltage stat that can be set for the contactor's coil current. This way the temperature generated by the coil of the remote contactor is nulled and the thermostat only reads the room temperature. The differential setpoint on a low voltage thermostat has a finer setting than that of a line thermostat, so a more accurate temperature is maintained.