There is one thing the FBI is very good at, and that's writing a compelling narrative. It's possible that there are highly competently people who know everything, but it's also possible there are moderately but not dazzlingly competent people who are really good at writing a story that feels complete and keeps one from asking questions outside the narrative.
Though, on second thought, I have to add that we don't know how many back-and-forth mail exchanges happened before they were able to come up with the officially published request.
Maybe they were just competent at working around excuses from Apple.
