Thanks for the reference!
It basically confirms what I already knew : ADS-B CAN transmit both barometric and geometric altitude, but only barometric is important (standard) for ATC.
If it is geometric (ellipsoidal NOT orthometric!) altitude that is in the Plane Plotter log - unlikely, but I have to confirm this - then everything is clear to me.
If it is barometric altitude - most likely - then I'm still stuck : barometric altitude can be expressed in geopotential feet AND geometric feet, as indicated in the implemented standard ICAO Doc 7488/3.