where's my CYA going

I had a post running on dealing with suspected mustard algae a few weeks back. My water has been pretty clear since and I'm gradually dropping the Cl level. I noted in my post that the CYA was 50-55 before going to shock levels for the algae. After I was done it was 35-40, with no additional water or rain added. So, about 4 days ago, I put 3.5 # of cya in a sock and put it in the skimmer. Which should have bumped me up about 20 ppm.

We were away for the weekend when the tropical strom hit the Fl coast. The pool gained about 2" of water and about 3 bushels of leaves and debris. It was in the pool about 24 hours when I returned home. I got it all cleaned up and put a gallon of 12% cl in to bring the cl up to 15. I checked the cya to see if I got the 20 pt increase I'd calculated. I was surprised to see that it was30-35. I know there is subjectivity to the test but I don't think It's that much. I've retested all of these values each time. I know the 2" of rain would dilute it somewhat but I didn't expect the low reading with having added 3.5 pounds three days previously.

So, I'm trying to figure out what's up. Obviously I need to add more CYA. I know about the biofilm consumption of it, but my pool is very clear now. Though this pool typically goes into winter with cya around 50 and by spring it is 0. I run the pool all winter but don't do much with the chemicals since the cl holds well in cold weather.

Any ideas.

If your pool has a hydrostatic relef valve (which I believe is code throughout Florida) it could have opened and let ground water into the pool since our ground was VERY wet as the storm went through. It's still be raining too hard by me to even check my pool right now and we are supposed to have more rain through the week (NE FL coast for me on the barrier island between the Altantic and Intercoastal and in between Jax beach and St. Augustine beach).

Evan, I'm just down the road from you on Anastasia island off 11th st. I've spoken to you when you used to be at Ace. Our ground isn't super wet because we are built into dunes. The pools isn't over flowed, It's about 2" from the lowest decking area. Water is nice and clear since the liquid cl last night was added. Where would a hydrostatic relief valve be, below the main drain? Our pool is about 12 years old I believe if that give you a hint on the valve. Guess I'll just have to pump more CYA in it after the rain is over and see what happens. I'm just puzzled since it seemed to start while I had the high shock levels going on. At least it seems that the mustard algae is gone, or perhaps just hiding.