Well that is very odd because that would leave the entire pass of Cirith Ungol open and ready for attack. Sauron must have been really sure that Aragorn had the ring at the Black Gate. He risked everything on that Aragorn had the ring at the gate. As Gamgee has pointed out Sauron could have thought that Frodo and Sam were diversions from Aragorn but, little did he know that they diversions were the other way around.
