@Frankee , thank you for the response. And it is known that the total of measures not always count correctly, you have solved that issue. See also Fix Incorrect Totals of Greg
As an outsider, I approached it as avg duration of outstanding inventory, which appeared to be Not brought back rented out material. After your clarification, I should expect something like rented out period in days (duration) per transaction per rental product per customer, whereby using the reporting date as end period when rentals not yet brought back, which could be averaged.
Anyway, if the report now suits your information needs, case resolved.