I looked some more at the stats. I counted 11 mature reviews in the “Reviews” section for one month in the picture, which is the same number the “Answer Buttons” section reported for the total amount of Mature reviews.
In the picture the large amount (166) of filtered deck reviews 15 days ago were done with rescheduling and here all not-suspended cards were reviewed at once. Thus, it seems like the “Answer Buttons” section only reports reviews done in the selected deck, while the “True Retention” section reports all reviews done on cards in the deck, regardless if the review was done in a filtered deck or not.