When you publish on KDP, you can earn royalties in nine different currencies. This can be complicated, and not needing to worry about it is one of the nice things about using Book Report. But sometimes you want to know how things are working under the hood.
So how does Book Report convert currencies?
Book Report has a currency conversion estimate for each currency for every month since January 2008. When generating a report, it converts to your requested currencies using the conversion rate for the month that the royalty was generated.
This means that changes in the value of a currency, relative to the displayed currency, will be accounted for in reports. The £10 you earned in February 2012 is displayed as $15.81 USD, and the £10 you earned in February 2017 is displayed as $12.47 USD. If you opt to have your reports generate in British Pounds, however, the earnings will be displayed as £10 for both months.
It isn't a perfect system, but it can't be. There's no way for Book Report to know what exchange rate you got when you deposited your cheque. For now, this is how the system works.