That was my thought, too. I'm guessing the group on the far right are the linemen, the group on the far left are the receivers, and the middle group are those that cover the receivers.

What I find most interesting is that there's a clear gulf between the (probable) linemen on the far right and the others. That middle ground is probably not best suited for any role - if you're chasing the receivers, you're going to be too slow, and if you're trying to protect or attack the line, you're going to be too small.