These categories were instituted 20 years ago.
As with all economic statistics there is an element of “fixing the bicycle while you are riding it.” They start collecting data, and over time the experience leads them to realize that things could be better done in slightly different ways. This is neither a sign of a conspiracy nor that the original statistical designers were stupid. It’s just that over time doing things, people learn more and come up with better ways to do them.
The paranoia about politically manipulated economic stats on FR is one of the stupidest and scariest aspects of this forum.
See my post at #7. There was a major revision in 1994, but the data on alternative measures of unemployment apparently go back to 1976.