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.
Likely Willie Cicci, as Pentangeli was high enough to be in the know. As a street boss in New York, Pentangeli would have been aware of the administration.
