The brand new 900s could be for AA. Some PSA 700s have more than 65 seats, eventually they will get dropped down to 65 seats and no longer be considered "large RJ's" which will create room for more 76 seat 900's for AA.

That will probably create room for 15 900's, plus if Delta is only sending 3 700s to Endeavor, and the rest are leaving DCI then that's another 27-28.