Frank Sinatra billed second?

I don't understand why this movie have two credit cast. one the open credit with mutch more sense what the second cast(reynolds,deluise,martin,davis jr,farr,henner, savalas and mcclaine the rest is secondary cast), and in end credits where Frank Sinatra figure billed second with only two scenes(????) in the movie, more jamie farr billed in third position upper dom deluise, dean martin and sammy davis jr. why? what is the reason for this?. imdb have the end credit cast in the info the movie with burt reynolds, frank sinatra and jamie farr as main actors, wrong situation for my taste.