Actually the tests were done in 1938 (editorial correction) but the report was issued in 1946.
Can you comment on my proposition that the "box" effect of the bound vortices, plus downwash, under wing surface and ground (runway) partially explain the resultant "compression"?
Sounds reasonable. We know that in free air the greater the vertical distance below the wing TE the lower the downwash [because of the greater 'radius' from the wing bound vortex centre - sorry I couldn't resist saying that
] so as the TE gets nearer the ground the more momentum the underwing flow has to shift to get by, so the bigger the force (pressure) it will need.
Different aspect of course, but wings fitted with endplates will need even more pressure build up because the air cannot escape sideways. This is limited though because there is no way the underwing pressure can build up beyond freestream total pressure.