Compaired both a 32" XT Combo O/U bbl and an XT 32" O/U bbl. Measured at the same points on each bbl.
The XT Combo bbl is .180 higher at the muzzle. At the mid bead it's .152 higher and .140 higher at the rear.
Hope this helps
Only guessing but with the rib on the un-single being higher I would imagine that the o/u's rib is higher so a person wouldn't have to mess with the comb. All three barrels shoot the same on my combo with no adjustments.. (80/20 approx.)