Backspace difference?

A lot of confusion regarding width and backspace is caused by the fact that wheel width is measured inside the bead/tire mounting surface and backspace is measured at the outer most part of the wheel. Strange really..
A 5.5" wheel is almost 6.5" measured at the outer perimeters.
One would think that a zero offset 5.5 wheel would have a 2.75 bs (half the width) while it in reality is has about a 3.25bs (half of 6.5)