Hey John... Glad to see you are still experimenting with the
vertical...
One answer is that you are shunt feeding a vertical monopole near it's
third harmonic tapped at about 1/8 wave above ground... Depending upon
shunt spacing and the diameter ratios it looks like you found the magic
spot for that frequency to transform to nearly a 1:1 match...
As one fella said to me long ago, I would rather be lucky than good,
any day...

denny - k8do