does CYA normal drift down throughout a swim season?

[aylad]

Besides the CYA being a subjective test, you're also going to lose CYA when you lose water (backwashing, splashout, draining rainwater, etc.)

[chem geek]

There is a slow breakdown of CYA from oxidation by chlorine, but for pools it's on the order of around 2-3 ppm per month (in hot spas it's more noticeable at around 5 ppm per month). Most of the loss is probably from the water dilution sources Janet (aylad) mentioned.