Mueller is aware that if he adds any new information to the report (his testimony), he will be prosecuted by the DOJ for Obstruction of Justice.
He is likewise aware that if he removes anything from his testimony, he will be prosecuted by the DOJ for perjury.
Mueller being prosecuted by anyone for anything lands in the realm of Powerball odds.
Yes, this is the answer. Because if you come up with a thought that was left out of the report, the first question would be, Why?
I should think Mueller will be very careful in this regard, and the DOJ simply and politely reminded him that what he says can and will be used against him.