There seems to be a bug at the bottom of the statistics screen, in the “Added” section where the average is calculated. You can see in my screenshot, that I am showing a one-year view. The total cards added in this year is 1,938. 1,938 / 365 = 5.3 cards/day, however the average is shown as 26 cards/day.
The graph has grouped the days into groups of 5, so I think the average is being incorrectly calculated based on these groups rather than individual days.
I am using the Mac verison of Anki Desktop.