First up, I'm not sure if their is 2 different standards from continent to continent.

But.....The groove Dia. of a barrel is not a critical dimension. Depending on the number of grooves/lands and the width of them a 7mm barrel could have a groove Dia of anything from .280" right through to 0.290".

The bore is far more critical when it comes to Pressure/velocity and the bore dia is more strictly maintained and quality assessed. Even said, the one thou difference you mentioned is a fair (not good, but acceptable) tolerance when it comes to a bore Dia.
Most factory produced guns bore sizes could easily have a +/- 0.0005" tolerance. Your higher custom barrel makers could hold a +/- 0.00025" tolerance (0.0005" {1/2 Thou} maximum variation).

The point I am making is that I find it hard to believe different continents would have different std specks on barrel dimensions when their is no doubt their sizes would be over lapping anyway from barrel maker to barrel maker.

It should also be noted that it wouldn't really matter at all when it comes to accuracy. A tight bore will result in higher pressures but you have to play around a fair bit to get a 'dangerously high pressure', for example tight 30 cal bores can measure 0.2965" and military bores can measure 0.302" (that's over 0.005" difference and both obviously work).

Hope this gives a little insight.

Tom.