Al Qaeda released an article in its English newsletter Inspire calling Iran's President Ahmedinijad a conspiracy theorist, and that he should stop blaming the US for 9/11.
This is more than just boasting by Al-Qaeda. It's not simply that they want the credit for the attack on the WTC, but specifically, they want to be distanced from the US. Inadvertently, Ahmedinijad is claiming that Al-Qaeda are collaborators with the US, rather than sworn enemies. If the US was involved in a coordinated way, it would undermine Al-Qaeda's anti-US credibility.
Even if Ahmedinijad claims the US wasn't coordinating actively with Al-Qaeda, but simply knew about it beforehand and prepped for the attack, it would undermine Al-Qaeda's credibility in effective attacks. If 9/11's mastery was really at the hands of the US, then Al-Qaeda has summarily failed to attack the US on its soil since 1993 (which was ineffectual at best).
The split is a sign that different Islamist groups, while anti-American, are far from being able to unite in the cause. It's a sign that US foreign policy--to keep the Middle East broken up and fractured--is working.