MyEnergi Zappi charge added: Integral not working

The integration for MyEnergi does not produce a value for the charge added (to the car) daily (only per charging session: see Zappi charged today · Issue #153 · CJNE/ha-myenergi · GitHub), so I thought I could use a Reimann Integral on the ct instead. Unfortunately it is not reporting the same power consumption as the MyEnergi app (eg yesterday app says Zappi charge added 5.7kWh, Integration (change yesterday) 6.23kWh).
The Integral is set up as a helper so I don’t know how to post its details. I’m displaying the results use a statistics card:

  - type: statistic
        period: day
        offset: -1
    stat_type: change
    name: Car
    entity: sensor.charge_added
Zappi charge added
19 hours ago
10.43 kWh
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TBH this isn’t the first time I’ve got a surprising result from a Reimann Integral.