Two weeks from complete-date to ii is pretty darn quick. (FWIW, I was 9/20 - 10/17) It probably just depends on what admissions officer gets your file and how high their pile of apps is.
As far as immediate accept vs. hold goes, I don't know that it's quite true that "very few" get accepted post interview. hopefuldoc did a nice little regression in last year's thread that seemed to show somewhere in the range of a 20-25% acceptance rate, post interview. (Which is fairly comparable to many schools I'm aware of--some are a little higher, some a little lower than that.) The hold pool-->waitlist path is a little murkier, since the number of waitlist offers has bounced around so much the last few years. Suffice to say, you have every reason to hope for good news post-interview, but your season is by no means over if you get a "hold" status. Folks can troll through last year's thread for more details.
And apologies for my pedantry--if over-analyzing this stuff annoys you just skip my posts, I just think it's worth putting the best available info out there so people can judge for themselves what to do and expect during the purgatory that is application year.
PS: "On hold for interview" means you won't get your up or down interview decision in the 12 week post-completion window, but may be considered again later in the cycle.