Heck! its you again... The reason is wear characteristic. You might have socks of the same design but of different states of wear. I being blind need to either buy all socks the same, which is not always possible, or to peg them together in the wash. This I do and use a sock bag in the wash to stop them straying which used to end up with odd socks as if the machine had been eating them.
Personally I'd rename all socks to something more obvious, Foot bags seems appropriate. Brian