Maybe it's the shirt you're wearing?
A couple of days ago I was testing FC and CC. At what I thought should be the titration endpoint I still had a very slight pink tint in the cylinder. Adding another drop of reagent didn't make it go away and neither did the second and third drop. I know just enough chemistry, and I know my pool, to know that something wasn't right.
I stepped outside the pool house into bright sun and lo! No pink. Ran the test again, outside this time, and the endpoint came when I thought it should.
The problem was the coral coloured tank top I had on. Inside the pool house its colour bounced off the white sticker on the chlorine test cylinder and made the test appear to be pink.
This brought to mind the day I walked on the pool deck and discovered green water. Talk about a heart stopper! There was NO reason for it to be green. Then I noticed that the green wasn't through the pool or in the pool but on the water, like some shimmery, sparkly ethereal layer of green. That one came from the reflection of my birch grove and the angle of the sun. In a few minutes, the green was gone, like magic.
Heads up. If your test doesn't come out the way it has in the past and if you have a good idea of how it should be, give some thought to the lighting and the environment in which you're reading the test. It may make a difference in your (perceived) results.