I've noticed something similar. Like you, I keep a manual record of sales and borrows/pages read in my own database program. I don't update the numbers every day, more like every 4-6 days, and any day-to-day losses would be invisible. When the new KU rate is issued mid-month, I go back and update all the numbers for the previous month at once. There are always reductions in numbers of sales - a few a day. However, the revenue from sales is almost always unchanged. My guess is that it's just end-of-month housekeeping, sweeping out any registered sales where the money wasn't paid, or possibly returns. I honestly don't think there's anything sinister about it.