[exactlysolved]

Sorry, but this is wrong. First of all the rates for publishing royalties are standardized and typically wouldn't be renegotiated on a song-by-song or project-by-project basis. So it is very similar to '10% of sales goes to the song writer'.

Equally important is the fact that a share of publishing means a share of all sorts of royalties which aren't negotiated case by case. For example for playing the song on the radio, in restaurants, on TV shows, etc. But even more insidiously, it applies to cover versions. So if Elvis Presley gets a 50% songwriting credit, he will then take 50% of the portion due to the songwriter on any version recorded or played live by anyone (who hasn't added the 'Presley magic'), at any time in the future.