Guess I should have added more information. I have a SWG, so my chlorine comes from that. I do not have a lot of calcium in my supply water, maybe about 150 ppm or so. My pool is new though and is a pebble finish. Could this be it?
The first weeks to month are the fastest, but it keeps curing at a possibly noticeable rate for 6 months to a year. After that, it's usually not that noticeable though it still technically cures very slowly.
CH of 150 in your fill water is significant. No calcium is ever lost through evaporation. Then, when you top off the water level, you are adding new CH. Unless you never add water to the pool, the CH level will continue going up every time you add water.
Once the curing slows down, with fill water CH of 150, it would take 3 full pool volume evaporations for your CH to rise that much...your talking maybe 4-5 years and even CH of 600 is manageable, as long as you keep your PH below 7.8 at all times.