The reason that they are called 3/8 is because back in the days before romex, when there was only BX and greenfield the BX came in 3/8", 1/2" 3/4 and so on, each requiring a seperate connector. However the 1/2 ko was suitable for both the smaller sizes, so it's threads and locknuts were used. Then the connectors evolved so that one connector could handle both 3/8 and 1/2 sizes. It's not so noticeable with bx anymore, but with greenfield there is an obvious difference between the sizes.
