Richard:
I can't do the chemistry but let me pose an alternate hypothesis. CYA SLOWS the breakdown of FC. The higher the CYA, the slower the breakdown. Now, let's assume that chlorine at a fixed FC and a fixed CYA performs a certain amount of neutralization, and has a certain breakdown rate (whatever it is--we know in high UV with no CYA it can be as little as 15 minutes even with no significant contaminants). As CYA increases, both the amount of neutralization (of contaminanats) and the breakdown rate from everything decrease. But.....do they decrease at the same rate?

If, by increasing the CYA and there for the "chlorine residual" we are actually spreading the work load among more chlorine ions, so that far more ions are doing the job, but have now a lower breakdown rate than contaminant neutralization rate, wouldn't we see a reduction in chlorine demand?

Just a thought from a non chemist.