You only add enough chlorine to keep your pool sanitized and algae free throughout the day. If your minimum is 4, and you lose say 3 ppm per day, you only need to start the day at 7. for example. The further you skew the CYA/FC ratio toward FC, the more of that chlorine is not bound to cya. It will burn off faster. So by being way higher than is needed, you are wasting chlorine and money.
That's not usual. Usually during the day one loses more chlorine at higher FC levels, CYA held constant. The loss is roughly a percentage loss. This is why the daytime losses when doing a SLAM are much higher than at normal FC/CYA levels. For your 19 to 4 in a week (which by the way would be an average of (19-4)/7 = 2.1 ppm FC per day) it may be a 20% loss per day such that you go 19 --> 15.2 --> 12.2 --> 9.7 --> 7.8 --> 6.2 --> 5.0 --> 4.0 where the initial loss is closer to 4 and the last loss is closer to 1. So are you saying that when it was at 19 ppm FC you did an accurate FAS-DPD test at that time and then 24 hours later and found it to be at 17.5?
So there is +/- 0.5 ppm for the 10 ml sample size it looks you are using, but it's clearly using up more chlorine initially at around 2 ppm FC dropping to 1 ppm or less at the end. The 2.5 ppm between Thursday and Friday is a blip but may really be 2 ppm and perhaps was a sunnier day. I thought you said that it dropped from 19 to 4 over a week -- the above would be from 8.5+10 = 18.5 ppm down to 9.0 ppm. Your chlorinating liquid might actually be more than 10% or your pool volume smaller than 30,000 gallons. With your numbers the deltas each day are as follows: 1.0 2.0 1.5 2.5 1.5 1.0 0.5