For any given load, those marks match up with some particular yardage, not necessarily even 100's.
For the physics geeks like myself, its just part of the fun to figure what those ranges are.
I bought a Leupold 2-7 with a LR recticle. The book said that said the marks match up with a 150 gr 30-06 at 2,3,4,500 yds.
The math for my 180 gr 30-06 worked out to 1,2,3,400 yards. I will probably never use the 400.