Well, 2.25 inches is 57.15mm exactly. 57.2mm is 2 thousandths of an inch large which is still with spec. But you're right, both units should be given with sufficient accuracy.
snip
It's still wrong. If you re-made your EDM go/no-go gauge today based off the 57.20mm as the nominal, rather than 57.15mm exact, the gauge would be worthless.
btw, I found the Wikipedia article. It was worse than I remembered. The metric and inch units with tolerances result in two completely different ball sizes. Something like this can easily creep into a spec, if the person doing it is not very careful (or knowledgeable on the subject), as this wikipedia illustrates.
Link:
https://en.wikipedia.org/wiki/Pool_(cue_sports)