Uh, me too!
I'm impressed by the quality of your investigation. It sounds like you have a 'difference voltage' caused by the notorious electrical villain, "the ground loop". (Wikipedia)
The problem is, there are all sorts of reasons why there might be a 1.4v difference between the potential at your pool and at your house's service panel. Your situation offers a GREAT example of what it's important to bond the deck as well. (I'm assuming that when you say "concrete pad", you mean concrete deck around the pool?)
Full bonding of all surfaces and gear around the pool would not eliminate the difference in potential between the house service panel, and the pool location, but it WOULD have eliminated any difference in potential between the pool and the deck.
A better ground at the pool may not solve the problem: there may be a 1.4V difference between the ground potential at the pool and at the house, and a better ground at the pool won't change that. If the voltage potential difference is a result of your electrical company grounding in the area -- say between a near by industrial or commercial feed with an unbalanced load, and residential grounds in your area . . . then putting a better ground by the pool may not change anything. Conceivably, it could even make things worse!
People forget (or don't know) that the power companies actually do use a common ground return to handle unbalanced loads; the "earth" -- dirt -- IS an element in these circuits. Almost by definition, the electrical companies are deliberately CREATING ground loops to balance these loads. If your pool is in the middle of an active 'balancing' circuit . . . you can add grounds till the cows come home, without solving your problem.
What probably would work, is extending your bonding mat to include every bit of metal on the pool you can reach AND adding a full buried wire loop around the pool that is also tied to your bonding mat.
(When he retired, one of my uncles was the chief maintenance engineer over all the hydro power network in TVA's Chattanooga region. So I called him and read him your post and my reply, and he essentially confirmed everything I said. He *did* note that these sort of problems never occurred until various forms of plastic pipe began replacing copper, iron & steel water pipes. When the power companies could bring that enormous network of metal pipes into their ground system, detectible ground loops were rare.)
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