Originally Posted by juicemouse
22.2mm (for "MTB" bars) or 23.8mm (for "road" bars) is the outside diameter where the levers are mounted. Though they don't state it in the specs you linked to, Shimano is referring to the inside diameter.
For reference, one of my drop bars had an inside diameter of 20.6mm, and a different drop bar measured 21.1mm.
You're right of course. Still, all the MTB bars and ends I've ever had were very thin walled. At 22.2mm OD I'd bet the ID would be right around 19mm. Also, the bars on my bent are 22.2mm and I use bar ends on it.
SB