I think that was a system devised by us for us  People were probably fighting about it in the past and that was the hardfast rule they came up with  Looking on the net for RSs the best I could find were these artilces    Basically they all say the same thing  There is no hardfast rule for deciding genre  Might be worth mentioning  When we discuss something that isnt obviously in a certain genre Naruto is shonen for sure isnt it then say something like xxx is classified as yyy by zzz to show that we arent doing the deciding