I’ve been meaning to post about this for a while now. I set up my GE Geospring hybrid / heat pump water heater using dedicated circuit monitoring back when the feature was in beta. The usage pattern clearly showed why the Sense native detection was underreporting usage from the heater. When the heat pump switches on initially, there is a slug of cold water at the bottom of the tank that needs to be heated. This reduces the temperature of the heat pump condenser coils, and therefore the discharge pressure at the compressor output, and therefore minimizes compressor power consumption. As the water heats, the discharge pressure and compressor power consumption climb. Native detection assumes that the power consumption of the device is relatively constant throughout the heating cycle, so it underreports usage after the cycle begins.
This also shows why keeping your HPWH tank temperature as low as practical can save energy, and why higher room temperatures reduce energy consumption by the heater.