Though Trichlor is cheaper, a faster and more efficient option for adding CYA is Dichlor. At least it dissolves right away. Anyway, I found that using Trichlor in my own pool took a little more than I expected to get the CYA to rise, but the turbidimetric test is clearly not precise visually and is probably quite prone to error at the lower CYA levels where other turbidity of the water itself would become even more important.

CYA presumably adheres to pipes and pool surfaces, at least somewhat, so some that gets added probably doesn't get measured in the water test.

Richard