Isn't there also a time factor in all of this. The required performance must have the aircraft contained within the required constraints for 95% of the time.
My underdstanding is that the time factor is very important. Fly a 60 mile series of legs at RNP 1 and (say 180Kt average speed and the required accuracy must be maintained for 15 minutes).
Fly a 3200nm RNP 10 leg at 480Kt and the required accuracy has to me maintained for 400 minutes.
The requirement is 10 times less accurate but the time it has to be held for is more than 20 times as long.
Therefore the probability of an error exceeding the limits is greather on oceanic routes and this has to be acccounted for.
I know that there are many other factors however that is my very simplistic view that RNP 10 is harder to do that RNP 1 - even before one takes the other factors into account - naviads, communications, surveilance.