interview times depend very much on the consulates involved... some are very quick; some quite the opposite
there are places here where you can check average times for your particular consulate

as for the rest - well, while Marta's point about what SHOULD be is very good, we're stuck discussing what IS, and I just wouldn't try to look for too much logic in this whole process, especially where dates are concerned... I have a vague memory that USCIS uses a contractor to move things from the service centres to NVC, so that's another variable in the mix; we just don't know if there's any very organised process to all this, or if they're playing catch-up there as well
my GUESS is, based on what we're seeing, that most July filers will have their NOA2 before Christmas, but when petitions will get to NVC, and THEN when interviews will be scheduled, is far more up in the air
sorry to not give you anything more concrete, but I just don't think it's possible