Why are pistol calibers measured with the bullet diameter, but bigger calibers the bore’s land diameter?

Question

E.g. The bullet of 9×19 is 9mm wide but of a 5.56 isn’t 5.56

Edit: 5.56 not 55.6

In: Technology

Answer 1

I know US calibers are sometimes measured by the groves but what about 9×19 and 5.7?