True Tire Heights?

[OLDFLM]

I'm looking for new wheels/tires for my 71 Firebird and for reference I took someone's suggestion and went out to measure my fender lip heights. With the current rolling stock of 255/60/15s and 275/60/15s the front fender lip measures in at 27.5" with the 255/50/15 at 26" and the rear fender lip measures in at 28.5" with the 275/60/15 at 27".

My question is that the 275/60/15 supposedly measures 28" according to most charts not the measured 27"... what's the deal? The tires are mounted on 15x7 Pontiac Rally IIs. When looking at new tires, say a 285/40/18 for out back, the charts say they measure 27", but do they really?

V/R,
Ty