Rally Wheel Weirdness???

[sick472]

I'm sure I am over thinking this, but how come my old CK stamped 15x8 Rally wheels seem to have a 4-1/4 backspacing?

My research indicates that they were all 4" BS. I am guessing that "back spacing" is a loose term and my brain only works in finite values? If I drop my straight edge into the outer lip, I get 3-7/8" and I'm pretty sure BS is measured from the outer edge to the hub mating surface anyhow and this yields 4-1/4" BS. Hmmmm???

My real concern is that I am using these old wheels to determine where I want my new wheels to live (~2" wider stance). If I'm "doing it wrong"...I need to know now before buying all new shoes.