Its possible that you are getting a small amount of heat from the earth. I believe the frost line is around 4 feet deep in New Hampshire and upstate New York so any pool that is say 6 to 8 feet deep will have some area below that line, Add to that the pool cover which can also hold in a little heat and there you go. I used to have a large pool in my London home and during the winter i would float a number of plastic beach balls on the surface so that if it did freeze it would crush the balls and not the pool.
Some pools should not be emptied as it will cause damage to the liner plaster .
