Kvom,
It is the little flats on the nibs that cause the problem with a digivern, they can't sit correctly on the curved surface of the bore.
But as you most probably know digiverns aren't classed as a precision instrument, and can only be relied upon to 0.002" (0.05mm). Even then they have to be held correctly to the job otherwise it can be a lot less accurate.
You are quite right on the use of the bore gauge, and the more feel you can get in your fingers, the more accurate they become. This is true of all hand held measuring, even the trusty mic can give the wrong readings if not handled and used correctly.
Bogs