The issue of Fredo being an underboss has been debated often here, and with no conclusive answers. Sure, he's listed as "Underboss" on the FBI chart at the Senate hearing. And Michael tells Tom, before leaving the compound, that he gives Tom power over "...Fredo and his men." But Fredo picks people up at the airport, runs a brothel and Mickey Mouse nightclubs, and is bitter about it--hardly the stuff of an underboss.

Likely guess: the FBI chart was wrong. That chart was modeled on the charts shown at the Senate Valachi hearings and were based only on what Valachi told them. He wasn't high enough in the Mafia to know who was really who. Perhaps the Corleone chart was based on info Pentangeli gave them--and he was wrong about Fredo.