One thing I never quite did understand is the logic behind bonding the rebar in a concrete deck since all of the rebar is surrounded by a fairly good insulator (concrete). I guess it is in case the rebar comes in contact with other metal that is exposed but I cannot think of any other reason.

Also, I think I remember from one of my power courses that ground, the dirt kind, can and does have changing potential from the house to the power company although they work pretty hard not to have that problem. So it is not inconceivable that the ground near the pool is at a different potential than the ground of the house/pad and both the house and the pool are not at fault.

The source could be from a neighbor's problem or a burried line that has become exposed. One possibility is 12v landscape lighting. It could be close enough to the pool and exposed to the soil to create some potential difference to the bonding wire. One way to confirm this is to test the voltage between ground at the pad and the soil near the pool vs ground at the pad and the grounding rod for the house. Unfortunately, you will need a lot of extra wire to make the runs.

It will certainly take some detective work to find the source.