I’m seeing some difference with a 240V HPWC, but not 20%. Here’s a charging session that lasted from 3:30AM until 4:25AM. DCM says I was applying 11.6kW pretty much continuously. The car says I added 10.16kWh. DCM put in 55/60h x 11.65kW = 10.7kWh vs 10.16kWh into the car. That’s about a 5% difference.
The numbers are from Sense and Tesla but integrated together in Home Assistant.