You guessed right the first time. Perhaps someone made a running change on the production line and moved the strong point down to the base. I see no engineering-related reason to do this. The purpose is to hold down the cabin roof against the upward pull of the halyards that turn 90 deg. at the collar bolted down around the mast hole. The force from the shrouds tends to try and compress the boat and raise the roof a little bit, also.
The way you have it now is the way it exists in the Valiant 40 and some other big boats with keel-stepped spars, afaik.
My '88 Olson has a short piece of rigging wire with a turnbuckle that goes to the opening on the mast with the T-ball socket. (Like the one in your photo.)
No matter where you put that wire, you always need some way to keep the cabin top solid. Some other sparmakers have a collar incorporating a bolt thru the spar for this purpose.
As far as un-stepping, I see no advantage either way. With the T-ball method higher up, you just mark the threads on the turnbuckle and unscrew it until you can pull the pin at the top and then rotate it and disengage it from the spar. I just did that when the rig was out of our boat this summer. It took a few minutes. (Taking the genny, the rod vang, and the boom off.... took me quite a bit longer...).
OTOH, everything I could take off or properly prepare reduces the yard time involved @ $65./hour...
Loren in Portland, OR
1988 Olson 34