Thread: Cartridge name vs actual size

There may be a simple answer to this but I've never understood, or just one of those dumb questions

Why do cartridge names not match the actual cartridge dimensions? the bullet itself is usually .2mm larger

for instance 5.56×45mm is actual a 5.7mm bullet, 6.5×55mm is a 6.7mm and 7mm rem mag is 7.2mm bullet