Yes, Reagan invented that. It's a contradiction in just 3 words.
If I remember right the powder bars are of different thickness. There are spacers in the aperture of the measure body to make up the difference for the thin one.
Work with them, it'll make sense when you see it. You'll also learn for certain which way to turn the screw to make adjustments to the powder drop and whether it's a linear function :-)
I haven't reloaded rifle in many years now so haven't had to change powder drop ranges recently enough to remember the details.