I have been out of the industry for a while so things could have changed , but QF always used to allow seating to be selected up until a certain threshold was reached - then it was closed off - this was IIRC to avoid the situation where a late booking family might be forced to sit apart because the only 4 seats remaining unselected where in different areas of the cabin (I think that there were other reasons as well , but as I say , I have been out of the industry for a while )