Perhaps the becoming 'totally public' of a place is contingent on the non-intersection of the sociality of some x number of checkins. I.e., if you have a party at you place but everyone who checks in is within 1-2 degrees of your social graph, then Facebook recognizes it's a well known place within a very local user subset. And doesn't make it public.

Also — time could be a simple indicator. If a place (i.e., your house) suddenly has 100 checkins over one day only to have no serious checkin traffic again for weeks, then that wouldn't be a completely public location either.