Refilling evaporate adds additional calcium to the water since the calcium did not leave with what evaporated. Even though the fill water is at a lower concentration that the pool water and the overall concentration is diluted, the calcium level still rises. An example: a pool's CH is 500 and 1000 gallons evaporate. The concentration is now higher (say 525) due to less dilution. We now refill the pool with a fill water that contains 150 ppm CH. Although the concentration is lower, and the now 525 ppm is diluted, you are still adding more calcium and the new CH level is 505ppm. CH slowly rises with time unless you get significant enough rainfall to overflow the pool or you pump water out and replace it.