They either have their allotment are currently being shipped in today or tomorrow.
What they did is guess essentially. Their "estimated" dates are based on how many phones per week/period they though Apple would give them.
So say they expected 10,000 black Plus 32gb version for launch to ship by the 16th. At 10,001 the dates shifted back to the next week up to say 20,000. Then 20,001 to the next week.
Now if Apple ships ATT 15,000 then great, ATT will send out 5,000 earlier than the dates they said. If ATT only ships 10,000 then ATT says, well we told you the 2nd week! If Apple sends less well some people see their dates drop back, but that seems to be rare.
The issue is ATT doesnt know before taking preorders. It's just a stupid system all around.