I wonder if the water temperature has something to do with this. Water solubility increases with its temperature, so if you put a ton of salt in when it was cold out, it might not have fully dissolved (even if you can't see granules with your naked eye). As the temperature increases, the undissolved salt dissolves and increases salt ppm. You might be able to experiment with this by taking a gallon of pool water and measuring the salt concentration while it's outside, and then bring it inside to warm up (or heat it on the stove).
Another thread on this:
Salt level keeps rising
