When you don't know how to measure the wheel correctly is when.
Lay a tape measure across the face of any wheel and it'll measure about 1" bigger than the tire it takes. The wheel has that lip to retain the tire side wall for a reason.
A = Where the tire actually mounts and that's 15" in this scenario
B = The face of the wheel and that measures 16" in this scenario
C = The lip that retains the tire to the wheel via the tire pressure