I talked to one of the "closed beta playtesters" at GenCon last year. It really sounds like all their real playtests are going out to the smaller, more controlled group. They are getting new packets every 2 weeks or so.
I'm fairly certain he wasn't supposed to say as much as he did, but I know at that time he was annoyed that they had only given the Warlock to them 2 days before they released the public playtest package with it in it. He made it sounds like the process is supposed to be: They get a packet, they give their feedback, if the feedback is mostly positive and no one points out any glaring errors then they release it as a public playtest. However, if a bunch of people complain, they redo that section and release another packet to the "closed beta testers".
It sounds like with the number of new mechanics and a nearly complete overhaul of the skills, classes, feats, exploration, and possibly more.....that this packet is going to be huge and so they are giving their closed beta testers more time to help them iterate.
It's frustrating, but I really do get the impression that all the real playtesting gets done by the closed groups and whenever they feel they've reached a milestone they release a packet mostly so they can get poll results from a larger audience.