Thanks everyone. I think I may have solved the puzzle. I'm leaning towards a combination of testing error and/or not knowing exactly how much dichlor/trichlor I really used.

First, I took out one of the 3" trichlor pucks and weighed it (I felt so "dirty" touching them instead of my bleach bottles!! LOL) - they're 7.4 oz - good estimate anonapersona!! But plugging in this new weight still didn't bring me up to where I'm seeing my CYA.

So then I decided to actually weigh the remaining buckets. Both the dichlor and trichlor buckets started with 25lbs each. Based on the remaining weights of each, I calculated that I must have used 170 oz trichlor and 169.6 oz dichlor. If you do the math you'll see that I was pretty close on my puck count, but apparently way off on my dichlor scoops. ... which is absolutely possible. During those weeks of using these products, with heavy bather loads, I found I was sometimes throwing in some additional scoops of dichlor to raise the FC level. I know, I know....what was I thinking?!!!

I know better now!

So when I plug all that into PoolCalculator (thanks again anonapersona for pointing out I had that tool all along

), I get an estimated CYA level of 59. Therefore my 50-60 measurements are right on the money. My guess is that the recent 70 reading was a fluke testing anomoly.

But heck, about a week ago the pool store told me my CYA level was 30!!! Based on all these measurements I just went through, IMPOSSIBLE!!

Bama Rambler - no puck feeder and nobody else adding pucks. I doubt the algaecide has any CYA in it either. I'm chalking this one up to a slight testing variation on my last test.