With water in the pool? I have no idea you would hear that no matter what

Temperature differences? Now you have a memory factor and other stronger factors contributing to differences as I would think that if one listens for 2 hours, the temperature will be pretty constant in that short span. I doubt 1 degree would affect anything enough to be audible.
Air density? Again, how much difference and effects of that density delta.
I just don't even entertain these thoughts so I don't notice it.
Yes, all of these would be a good factor, especially the background noise floor. 57 dB is huge. You add maybe 80 dB on top for a peak.