I know what you mean and I agree with you, but It sometimes makes sense anyway, in the movie and the book Puzo does not explain how Michael came back to America. We can considered as a cover of a missing story, you know there are many missing parts and year in the Corleone Saga, when I get the chance and arrange some details I am going to make a timeline to cover all the missing parts with things from the webpage, books, movies or whereever I can get info from.