Haven't you heard? Sequel numbers are passé. It's all about Title: Subtitle now. That way you aren't so obvious about your franchise milking.
See AC: Black Flag, Killzone: Shadowfall, Infamous: Second Son etc.
1) It's called Assassins Creed
IV Black Flag, it has a number attached to it. In fact people were complaining that they decided to attach a number to it and it would've been better off that they had just kept a subtitle (without a number) instead. Should've used a better Assassins Creed example, like Brotherhood and Revelations.
2) KZ and inFAMOUS are not milked, and having a subtitle instead of a number makes sense. The games are designed so that no knowledge about the story from the past games is needed. They're both starting up new stories with new characters/conflicts and are perfect entry points for people that haven't played any of the other entries in the franchise