I'd be surprised if we only went gamma in September. I mean, the growth from alpha 3-4 in a month is extraordinary, 4 months I reckon the game could be done (though perhaps not 100% bug free)
Software is not necessarily developed in the same order in which it is released.
In a project of this size, it almost certainly isn't a matter of everyone working on the next release all at once; I would expect there to be different devs/dev teams working on different aspects of the game, each of which will have their own priority and so on.
Some things currently being worked on may be included in near releases if they're almost ready, other stuff might not be due for a while but will require a lot of man hours to implement so they need to start those things now, and so on. Other smaller features that aren't time critical will be squeezed in as and when various staff find the time to add them.
Despite what we have seen of the game so far appearing to be coming along nicely, that doesn't mean there aren't going to be bumps in the road in the future. This is probably the reason why no firm release date has been set yet, because behind the scenes there must still be quite a bit more to do that hasn't yet been accurately estimated in terms of time required.
TLDR: patience, my young padawan.
