Would you say the depth is below the frost line? Did you notice if they isulated the pipes?

I looked up the equipment you have and this is a very nice setup!
According to one source I found the heat pump is actually made by ClimateMaster. If that is correct, here's a link to the installation manual.
http://www.climatemaster.com/downloads/97B0045N01.pdf
And a programable heat pump thermostat
http://www.climatemaster.com/downloads/ATP32U03.04.pdf
If this is your t-stat you have a fair amount of control over the heat strips through this interface.

There are two possible modes (algorithims) for the control loop, "Proportional Integral" or "Differential". Proportional Integral is the default mode. The installer would have set this option.
If the control loop is very "tight" there will be very little deviation from the set point but the system may need to use third stage to keep it in the loop in the time alloted by the algorithim. This may be what you're seeing since your electricity use seems very resonable. I'm thinking the heat stips may be energized for a vey short amount of time to keep it in the loop.

You have a two speed compressor so low speed is stage one, high speed is stage two and high speed with the heat strips energized is stage three.

You can control the amount of time the system has to meet the heeting demand with the heat pump cycle through the "Smart Heat Staging" option. I would try increasing the time interval to see if the system can meet the demand with stage two of the heat pump cycle.

"SMART HEAT STAGING
This will control how the thermostat to engages 3rd stage or auxiliary heating when needed. Options are ON or OFF. Default is OFF. When the option is set to ON, the time range for this option is 5-120 minutes in 5 minute increments, if set to ON. 5 minutes is the default time. This is the minimum amount of time after temperature conditions for activating auxiliary heat are met, before the electric heat is engaged."

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