Think I know why the dashboard is calculated wrong…
The Shelly 3EM don’t take into account the energy sum, but instead only report individually all 3 phases (used and returned).
So if phase A and B use each 500W for one hour and phase C returns 500W for one hour, the total is a usage of 500Wh, and not 1kWh used and 500Wh returned.
Apparently Shelly announced last year a new firmware to address those summing smart emters, but nothing on the horizon yet. Seems this is the only way ATM: