During the spring and summer months, our pool (an outdoor pool) WAS maintaining pH readings ranging from 7.4 to 7.
As the weather has cooled (sometimes REALLY cool) pH readings have consistently bee at 8 or more and we've been going through tons of muriatic acid.

Summer water temperature is 78 to 80 degrees, and in the winter it's kept at 68 to 70 degrees
To paraphrase Slim Pickens, what in the wide world of sports is a'goin' on here?