There are any number of things about the Godfather that seem curious or inconsistent. For one, what was Tessio's first name - not in the book and not in the movie? Sal? That's the name Tom uses just before they take Tessio for a ride at the end of the movie.

And given that the Corleone family was the biggest in NY, why only two Capos or three if you count Sonny and his regime?

Michael did not have an underboss probably as a functionof this personality. He had alot of arrogance.