my impression:
the tip oring seals against the breech of the barrel, sealing the air so that the air transferring from the bolt goes directly into the barrel. so no loss of air.
as for a cut out to access the lug, i don't know. i think its more of how its always done, or even because of the design, it might lead to a weakening of the bolt. i do know that since the 97 which had the cutout for the lug through the top, where earlier cockers needed to have the frame removed; that the bolts were a carry over.