CYA is an additive that does not come in tap water. It does not go away, so the amount you add cumulates. When you backwash, vac to drain, etc. it removes some and when you add water it gets diluted a bit.

If it is in the chemical (6 in1) that you are using, it is being added. Don't know what rate it is being added at, but you can get a test kit or bring your water to some pool stores to get it checked. If using a store, I suggest going to a couple or go back a couple of days apart to ensure they have the same measurement. The stores can and do mess up tests sometimes. I believe this forum recommends a typical range for home pools from 30 to 50 ppm.