The reason that FISA warrant requests have such a high acceptance rate is because a federal prosecutor wouldn't ask for one unless they were certain it would be approved. They don't have to file a request every time a federal agent wants a warrant. They have the option to say "You need to gather more evidence".

They say a good trial lawyer won't ask a witness a question without knowing what the answer will be. FBI lawyers know the law and they know the judges, they won't just take shots in the dark when it comes to obtaining warrants. It's like asking why you have a 100% approval rate for your bank card purchases. If you know that you don't have enough money, you won't try to buy something.

Keep in mind that it's difficult to become a federal prosecutor and the job attracts intelligent people. The kind of lawyer that you see advertising on the side of a bus or a local DA trying to make a name for herself will try to push the envelope to get as much as they can. But high level corporate or government lawyers rarely sign their names to anything that isn't legally rock solid.

I just saw the Obama interview and he only hinted at this answer. As a Harvard trained lawyer he understands the system, but as POTUS he can't always be candid.