This is my first season as a pool owner. I've been testing my water with the TF-100 kit. The weather here in west-central Georgia is quite hot. All season, I've been keeping a CYA level of about 50 (as was recommended by my SWG manual). With a CYA of 50, I had my SWG set at 60% with a pump run time of 12 hours. This was maintaining the FC between 4.5-6.0 depending on conditions.

About 4 weeks ago, I bumped up the CYA to 70 and lowered the pump run time to about 10 hours. Curiously, for the past couple weeks, the FC has been hovering around 7.0-9.0. About 2 days ago, I lowered the SWG to 40% to try to keep it back in that 4.5-6.0 range but the water just tested with a level of 8.5. I'm using the DPD test to test FC/CC. CC has been at zero all season.

My question is whether the change in CYA from 50 to 70 is the reason why the pool seems to be able to maintain such higher FC levels with a decrease in pump run time.

Yep! That's why we recommend 70-80ppm CYA around here. Despite what the mfr's say.

Your cell will last longer too. Win win for everyone (except the cell mfr's). Yay for CYA (it's not so evil we can't give it some love every once in a while eh?).