The content of the tape- That al Queda were surprised by the level of destruction, and that the suicide bombers were not aware of the nature of the task, seem to be positions Osama would wish to put forward. They call for restraint in punishing al queda and their members.

Would the Americans of published a tape that contained only those messages? - i think not
Therefore he admitted to the attacks knowing that the Americans would find it hard to resist publishing such material, but compromised the authenticity by wearing a ring- (which he does not and furthermore somthing which is uncharictaristic in Islam)

This left the U$A in a predicament. Whether or not to publish somthing which directly contradicts the western line,- (all bombers trained for months and were fully aware of their task.)and also has dubious credibility due to the ring, but crucially in which Osama admits to the attack.
They must have been aware that the ring issue would have had more meaning for muslims and would tend to make them disbelive the authenticity of the tape, and that it is also precicely those moderate muslims who they need to win over to their phoney war. However the yanks could not resist putting out this crystal clear admission of guilt and thereby dividing those who know that muslims don't wear rings (mainly muslims) and those that dont know that (pretty much all Americans)into camps of -video must be a fake /and- he even admitted to it on TV
All in all an acomplished piece of media manipulation by Osama that the Americans could not resist being complicit in.