The interesting question then: Is the improved performance of
vertical antennas over horizontal dipoles on 75 meters at DX distances
due to a combination of direct radiation plus radiation from the
ground in the area of strong ground wave strength out hundreds of
meters? Is the ground wave leakage providing additional low signal
strength in both transmit and receive?

Gary - N0GW

That's always interesting - and usually without a good explanation.
I don't know how many times I've switched from one anntenna to another just
to have the quote"inferior in theory" antenna get better results than the
"better" antenna.
Antennas are magic. There may be a metal building or some
near-but-out-of-sight tv antenna mast or tin roof or aluminum-foil-lined
insulation giving some directivity.