How wide is a 2.2" MTB tire? I brought my MTB into reputable local bike shop to swap out a worn 5 year old tire.  Front and back had 2.2" tires that were actually 2.2" wide.  Mechanic says he has a new tire of the same kind ("Continental 29 Mountain King 2.2") in stock.  I was thinking to go wider but say ok and ask them to install.  New tire says "2.2" on the side, but installed and inflated it is 1.8" wide and visibly narrower than front.

Are tires sized like lumber, you take 0.5" off from nominal?    

It otherwise seems fine, aggressive tread, etc.  Should I care?  Am I over thinking this?  I'm just a fitness XC rider on single track trails in all kinds of weather.
MTB tyre sizes are very variable, and the only way to be sure what size you will end up with is to read online reviews and/or see it mounted on a wheel in person.

mattnz is indeed correct in his comment that size can even vary between models from the same manufacturer.

The Conti Mountain King 2(and the XKing that shares the same carcass) in 2.2" flavour are well known to come up small and are generally considered to be a 2.0/2.1.  No need to be concerned though, as they are generally well thought of as XC tyres.

With these two tyres i'd be more concerned about getting the correct rubber variant of rubber compound (either the PureGrip or BlackChilli) than the true size of the tyre.