Actually, Brian, those numbers are incorrect.
Backspacing with zero offset:
7" wheel width -> 4" BS
8" wheel width -> 4.5" BS
10" wheel width -> 5.5" BS
It is confusing because the width and backspacing measurements are taken at two different locations on the wheel.
The width is measured between the inside of the rim flanges where the tire beads seat.
The backspacing is measured from the wheel's mounting surface to the innermost part of the wheel which is the other side of the rim flange.
Since the flange is about 1/2" wide on most wheels, that accounts for the apparent discrepancy.
So, a 7" nominal width inside the flanges translates to about 8" overall width. Split it in half to find the backspacing at zero offset and that is 4". The positive offset of 12mm adds about 1/2" for a total of 4.5" on all 15x7 or 16x7 factory Ranger wheels.
Almost all recent factory Ranger wheels, whether they are 15" or 16" diameter, steel or aluminum, have the same common dimensions:
The only exception in recent years are the base XL steel wheels:
Here are my factory 15x7 wheels:
This diagram shows where the measurements are taken:
This is a good cross reference for offset, width and BS: