My FAS/DPD tests is showing crazy numbers. Tested this morning and FC was 17.5. Doing a SLAM so want the FC at 20. Put in 1/2 gallon this morning to bring FC to 19. Tested again when I got home from work. FC was 14.5. Added 2 gallons of 10% chlorine to bring it back to FC 20.5. Just tested and it is FC 27.5. Been using pool math and have everything dialed in correctly. What am I missing? What could be going on?