One of three things is true: 1. The FBI is lying to save face, 2. Someone -- but probably needs to be some people -- from Apple is willing to help them, 3. There is an undisclosed vulnerability in iOS which Apple is unaware of. I find it really hard to believe that someone would disclose a zero day exploit to the FBI without charging significant $ for it. Am I missing something obvious?
The possibility that the Bureau is just plain dumb is not on your list. They are on record, many times, and in front ofa very serious congress, being very very stupid. Though I do not believe they are being stupid here, the possibility is not an edge case and should be considered.