Energy meters in 3-Phase configurations show wrong amounts

[Tho-Mat]

My home is connected with 3-phases to the grid.
I have a saldoing grid-meter, that means if i export energy on some phases and import energy on other phases
the meter only counts the difference.
So, if Phase1-Energy + Phase2-Energy + Phase3-Energy is >0 at a time, the import meter rises.
Or, if Phase1-Energy + Phase2-Energy + Phase3-Energy is <0 at a time, the export meter rises.

I always was wondering, why the powerwall values METER_X_LifetimeEnergyImport and METER_X_LifetimeEnergyExport do not match my home grid-meter.
On the other hand the values shown in the Tesla app do, with only a small error.

Example: 2024-09-14
grid-meter of the grid-provider:
import: 0,41195kWh export: 20,5937kWh
App:
import 0,3kWh export 20,5kWh
Meter_X:
import 4,606kWh export: 24,817kWh

So what does Tesla do in the app?

I recorded the meter values every 5min and then calculated the difference of import - export.
If this is negative, then I assume an export of the kWh difference.
If this is positive, then I assume an import of the kWh difference.
I now made a summ over all positive values of the day and get 0,293kWh for import.
Also the summ over all negative values gives me 20,504kWh for export.
I do not know if Tesla uses a 5 min interval, but using this gives a good approximation.

P.S.:
This may also apply to (3-phase) Y_Meter or PW3.
The actual METER_X_LifetimeEnergyImport is not senseless.
In the example above you can see, that in an ideal island configuration
I would at least have consumed round about 4kWh more energy.
Unfortunately the METER_X_LifetimeEnergy does not show the total amount of imported/exported energy.
When i look at the three METER_X_CT?_InstRealPower values
the total amount of imported energy is 10,7kWh and 30,9kWh for exported energy.