Angle of Attack and Angle of Collective Pitch
Perhaps we are all envisioning the same thing, but in a different way.
Nick said; "The pressure below the helicopter IGE is the same as the pressure below the helicopter OGE."
Assuming that the helicopter is hovering in both of the above situations, I don't think that anyone disagrees with this statement. Nick has also said that the angle of attack will be the same in both situations and nobody probably disagrees with this, either. The thrust will also be the same.
What is different between hovering OGE and IGE is the angle of the collective pitch. In other words, the effect of the ground is that of allowing the helicopter to hover with a reduced collective pitch (plus a reduced rotor drag and a reduced power).
The question is; 'What is the cause of this effect'?
Pressure is a potential &/or partial answer.