Hoping this clarifies the process:
--The bootfitter adds material to the lug to make it too thick.
--...then shaves away material on the top, or on both the top and the bottom, with the end result being that the top and bottom end up "tilted" to the same degree.
--The space between the top and bottom of the lug, from inside to outside of boot, is the same thickness across when done.
--And that thickness fits the binding requirements.
--The bootfitter adds material to the lug to make it too thick.
--...then shaves away material on the top, or on both the top and the bottom, with the end result being that the top and bottom end up "tilted" to the same degree.
--The space between the top and bottom of the lug, from inside to outside of boot, is the same thickness across when done.
--And that thickness fits the binding requirements.