When I first got my pool around 7 years ago, I used Trichlor pucks/tabs in a floating feeder. The pool has a mostly opaque pool safety cover and has an oversized cartridge filter so no backwashing and only needs cleaning once a year (so minimal water loss). I also had a pool cover pump, no summer rains, and put winter rain water from the cover into the drain and not into the pool (at that time -- now I dilute the pool water with winter rains). My daily chlorine usage was low at around 0.7 (to 0.8) ppm FC per day though was as high as it was mostly due to 88F average pool temp (via solar heat). Nevertheless, after starting out with 30 ppm CYA, I got to 150 ppm CYA in just 1-1/2 seasons (about 10 months) of use when I began to get problems with cloudy water and higher chlorine demand (a nascent algae bloom) in spite of using PolyQuat algaecide, though only every other week. This was mostly because 10*30*0.7*0.6 = 126 ppm plus my initial 30 ppm. So this effect is very real and it does occur in real pools, but my situation had virtually no water dilution.
In Charlotte, North Carolina, you get rain pretty much all year long ranging from 2.95" in April to 4.39" in March. The annual rainfall is 43.5" or around 3.6 feet. If this regular rain overflows the pools, then this continual dilution in a pool with average depth of 4.5 feet is around 55% annually. If the pools typically have higher CYA levels, then they might have fairly low chlorine usage and your swim season may be shorter as well. Let's say it's 1 ppm FC per day over 5 months. That would be around 90 ppm CYA so with the continual dilution the CYA level might never get that much above 150 ppm. I also haven't factored in any dilution from backwashing sand filters (especially in smaller pools where the volume of backwash is a larger percentage of pool volume), from splash-out, nor any CYA loss from oxidation by chlorine or bacterial conversion to ammonia over the winter (though that would show up as very high chlorine demand upon spring opening).
Another factor is the ozone since that can sometimes lower the chlorine demand (sometimes it increases the chlorine demand -- depends on the bather and organic load). Also, the pools may have had rather low chlorine levels and therefore lower daily chlorine usage than I indicated above -- perhaps it was only 0.5 ppm FC per day (so 45 ppm CYA over 5 months). Pool water temperatures are also a factor where cooler temperatures (closer to 80F) have a lower chlorine loss rate (slower nascent algae growth; less outgassing; slower oxidation of CYA). And of course I've been assuming Trichlor-only for the chlorine -- if you use a mix of SWG and Trichlor, the amount from Trichlor will be lower than I assumed.
Richard