The ops engineering cells in the various airlines maintain continuous records of how individual airframes vary from book performance values and how these variations vary with the passage of time. It is then relatively easy to come up with correction techniques so that crews are still provided with adequately accurate performance data for flight planning. While I am not familiar with this approach it is just another way to dress the bird up so it still looks fine and dances nicely.
Been the story for many, many decades, now.