Several nights ago, I topped my FC off to 8 ppm.
I usually do it every other day. It was reading 2.5 which is abnormally low. I tested it twice from water at different places in the pool.
I added 10 cups of 8.25 bleach. So far so good.
The next morning I tested again and read 14 ppm. I tested twice more and read the same thing.
I had used a new bottle of bleach. I shook it and it had about what would be expected with 10 cups gone.
I had not mistakenly added bleach twice the night before.
That night it read 9 ppm. That is a loss of 5 ppm which I have never witnessed before.
Over the next few days it behaved as it always has- no strange readings.
To save my life, I can't explain what happened. Bad tests is the only possibility and I double and triple checked them.