[gfo]

You use standard pressure (29.92 inHg) above transition altitude, which, in the United States, is 18,000 feet. Pilots wouldn't be changing the altimeter after climbing past this point, and would start using local values once descending through it again.

Of course, your initial point is still correct: there could be slight variations if using those local settings and getting different values, but you'd only see that below transition altitude.