iit is different because 3/9 multiplyed by 6/9 would be 18/81
when adding 3/9 by 6/9 would give you an answer of 9/9 which would give a simplified answer of one.
in conclusion while multiplying everything the denominator and numerator are going to be multiplyed together and while adding 3/9 with 6/9 would give you a lower number with a more reasonable approch