Q: hey why is it called a 9 and not a 2? like E9, even though you might be playing it an octave above the other notes
A: 9 generally is telling you that the 7 is there too. but if you say 2, there is no assumption that the 7 is there. so if you really want the 2 and 7 to both be there, just saying 9 is kind of like a one shot deal. save you some time there