I think there are different houses because only 9 students live in the house of Anubis and the school has many more students. Also in season 1 when Nina joined and the taxi was on campus she specified House of Anubis which means there must be other houses also.
there are other houses. i see your pic is from season 1 but im on season 3 and later in the seires it say that there ia anubis house, mut house, and isis house. there is another but i forget the name of that one. good question! hope this answer helped :}