Some things just aren't simple!
For my money Ground Effect has to do with how horizontal the air is as it approaches the wing or rotor. This affects how vertical the Lift Vector is, and so how big the induced drag is. But the actual details are significantly different for aircraft wings, helicopter rotors, and aerofoils spanning wind tunnels, even though the end result is similar.
Vehicles specifically designed for exclusive GE operation could also show effects that we don't even notice in normal aviation.