Should CYA be stable or can it vary?

[elanmike]

The CYA level in my pool has been 70-80 for a long time and I've not had to add any Cyanuric acid. I just got back from a business trip and tested the pool for the first time in 4 weeks. Surprisingly, everything was still pretty good (pH a little high, but SWG kept chlorine good) except CYA had dropped to 55!
Should CYA be pretty stable, or this type of drop normal?

 

[Bama Rambler]

It's not uncommon for CYA to drop over the season. Have you had a lot of overflow? If so, that would explain it.