The companies don't do it this way, really. We do it as a good enough way of checking the size of a 5 lug bolt pattern. The diameter of the bolt pattern would be 4.5", for example, but when measuring from lug to lug, you are not measuring the diameter, because the lugs are not on opposite sides. By measuring to the outside edge of one of the lugs, it adds a little to the measurement. Just enough to get a close enough measurement of the bolt circle. If you measure from center to center of a 4 lug bolt pattern, you would get the correct measurement.