And yet -- why were a GK's hands/arms NOT included when the GK was a defensive player then?The explanation was not in the law when the wording first excluded hands and arms. I'm saying the reason now given has only been provided years later. USSF had this ten years ago:
USSF answer (October 1, 2008):
The Law is quite clear about this. Any part of the body that can LEGALLY play the ball is considered when the referee looks for offside. That excludes the hands and arms, as they cannot legally play the ball. The same is true of the hands and arms of the opposing players.
I'd never heard this "new" rationale until this FAQ came up with it and I see no evidence that the "help the AR" argument was why hands and arms were excluded in the first place.
This is likely yet another case where the USSF made an assumption... and it was the wrong one (see their "Advice to Referees" for some excellent other examples of such).