Everyone that passes the tests together won't get the same interview timeframe. They are hiring on a month-to-month basis, with new hire classes starting every month. I tested at the same time as 2 other friends of mine. I got in first, one got in a month or so later, and the 3rd got in 2-3 months later. Once you interview, it's another wait until you hear about a job offer, which is definitely pending you clearing the extensive background check and drug test. You may get a "start date" of, for instance, January 2015, but if your background check takes a while to clear, they will push it up a month to February, so look at it as a "tentative" start date.
It's just how their process is going, it's frustrating, but nothing you can do.