By default, Book Report assumes that page reads will be worth the same amount this month as they were last month. On the Settings tab, there are three options if you'd like to switch it up:
With the most recent rates. This option assumes that every marketplace will pay as much per page as it did last month.
Assume a drop is coming. This option assumes that every marketplace will pay less than it did last month. How much less? It finds the biggest drop for each marketplace that occured in the last 12 months, and assumes a drop slightly bigger than that one.
With a custom rate. If you want to keep things simple, you can still set a single custom rate that applies to all marketplaces.
Book Report only estimates the value of a page read when it doesn't have the exact numbers yet. Every month on the fifteenth, KDP releases their Prior Months' Royalty report for the previous month. That report contains the exact royalty earned from each page read, and Book Report integrates that data and stops using an estimate for that month.