I believe the concept is that you're supposed to take items from whichever list has the smallest thing first until there are no more items left. So, if it had been:

[6,5,2] + [3,7,9]  =  [3, 6, 5, 2, 7, 9]

But, it does leave some ambiguity:

[3,3,2] + [3,1,1]   = [ 3, 1, 1, 3, 3, 2 ]  or [ 3, 3, 2, 3, 1, 1 ]