You probably want to read here in the Input Output reference about the WaterHeater:HeatPump:PumpedCondenser which is what I assume OS uses when you call for Waterheater:HeatPump object.

You see, OS is creating in the background few different E+ object in order to fulfil your demand. Evident for that can be seen when you select the component in the Thermal Zone tab and look at the Edit tab on the right.

If you roll down you will see the different OS object created. One of them is OS:WaterHeater:Mixed the one where Heater Thermal Efficiency is.

So that refers to the secondary heat source of the water tank in this configuration after the first source which happens to be the OS:Coil:WaterHeating:AirToWaterHeatPump.

Since the tank heater is not a Heat Pump it's efficiency can go as high as 1. If you don't need the tank heater which is default to run on Natural Gas you can hard size it to 0 W.

If you roll further down you will find the OS:Coil:WaterHeating:AirToWaterHeatPump object where you can change the coils COP or even further down the efficiency curves.