On a first fill your pool water has the same hardness as your fill water.
Then pool water evaporates and the calcium stays leaving you with a lower volume of water with a higher concentration of calcium hardness. The only water to get back to the original volume and concentration of calcium hardness is to add water with no calcium in it at all, like distilled or RO water.
More practically, you'll be using more spigot water to refill the pool, this adds more calcium. Thus it's expected that calcium hardness generally increases.