The ground here in new England is about 45 degrees Fahrenheit ,just a foot or two under the surface. Most nights are cooler, and this means that a lot of heat is being sucked away from the water by the ground.
We also have a very rocky soil and even some broken glass under the grass.
This has cushioned the bottom of the pool from the rocks, and has allowed the 6,000 gallons to gain as much as 4 degrees in one day. Currently the pool is staying in the 80s without any extra heating, just the solar cover.