Glamorgan the app (and portal) gets the grid import from your EM115 and grid CT. The EM115 can’t readily detect very small values of energy flow (say less than 50W) but flows above the small value are detected and registered in the relevant import or export register, read and reported by the inverter to the cloud. It’s this inverter input that is used in ECO mode to control the inverter to minimise grid import/export.
For small differences in Smart meter vs app values, I’d question if the EM115 CT wasn’t clipped in firmly. But from the magnitude of the difference between your values, I’d question whether the CT was fitted in the right place. It should be installed ideally between the DNO supply fuse and the energy supply meter. There can sometimes be a Henley block fitted between the smart meter and consumer unit to take a supply off to another consumer unit which would skew readings.
Whatever the reason, and error in excess of 100% can’t be right.