Interesting indeed. Controlled test/observations from Marcy would be very insightful here.A few things come to mind. Is the change in reception with height more less prominent at different reception frequencies (i.e LF, MF, HF)?
Is there any interaction with the feedline picking up signals, or contributing common mode signal?
If local noise fields are the enemy, close proximity to the ground may be beneficial, similar to what others observe with the LOG (loop on ground) or BOG (beverage on ground).
Sources of noise, and how they get into your radio are something of a fascination at this listening post