False CYA reading?

17x33 inground, 22,000 gal., vinyl, w/Hayward EC65 DE filter
At the start of the season (early June) I opened the pool and raised the CYA to 40-50 with 5lbs of stabilizer. Haven't had to do much else except adjust the pH once and it's remained at 7.2-7.4. CYA has remained stable as well. Chlorine I use is the 12.5% from the blue carboys I get from the local Mom 'n Pop pool store, same as I've used all season. As far as I know it's only chlorine with no additives.

Until this weekend.

Friday night it was still 40-50, FC was at 6ppm, CC at 0. Had to brush some algae off the sides and planned to shock and dose with algacide Sunday (second dose this year). By Sunday afternoon CYA is reading 70 to 100! Tested three times with the same agent I always use using two different graduated tubes. The 70 reading was really only because I strained to see the dot. I really try to test CYA under the same conditions each time knowing the possibility of different lighting conditions skewing the results.

The only 'chemical' added between Friday night and Sunday afternoon was a ferocious thunderstorm we got Saturday afternoon. Other than that nothing was added. I did do a thorough backwash this morning.

Anyone offer a reason why my CYA would have jumped this way?

Bewildered,

C.

probably just a difference in testing--unless you've added CYA, or had some shock wash into the pool during the storm, it wouldn't jump.

Janet