well, if the official release tiy has said will absolutely-be-in-2013-no-matter-what, then putting out the beta in september or october doesn't give them a whole lot of time to finish all those additional things for the final release.
and although i'm stating that, in reality i actually don't think it'll actually be a problem. chucklefish is obviously not worried; they decided to put starbound in a UK convention, which means they believe that their game is at complete enough to let people preview. the progress they've made on the game in the past three months is fantastic, too. i think we can expect the beta fairly soon, if not late august then possibly mid-september.