That does seem a bit high for a drop. Losing half of the chlorine over a day is more typical -- so 15 ppm to 7-8 ppm would be more "normal". Then again, I could be wrong about those half-life numbers. They seem to be correct for most pools, but there's no arguing whatever it is that you are seeing in your pool. It's also not like you are in south Florida or the middle of summer. Maybe there is in fact some algae in your pool and it is growing more during the day and consuming more chlorine, but typically if it holds overnight then this means the algae is gone. Sorry I don't have a good answer for you.

Richard