Interesting. I noticed the reagent you're using is different from the one I'm using, Taylor R-0014. Perhaps this explains the difference in our experiences?

Under 10ppm FC, I can take my time reading the result. It doesn't change near as fast as it did for you until my FC gets above 15ppm FC or so.

It would be great if Chemgeek would pop in and give his input.

To the OP - my apologies also for the sidetrack, though it may relate to what you are seeing. How long to you let the pool circulate after adding chlorine before you check pH? What's the FC level at the time you check pH? As you can see by our discussion elevated FC can give a false high pH reading.