Imagine that instead of two numbers you select a million numbers, and apply a zip algorithm to that sequence to obtain a file then you map the first byte of that file using (first byte) mod 9 + 1, intuitively there must not exist any correlation between consecutive tries of this strategy. Looking for this example I realized that instead of using n1+n2 you should use a non commutative operation to improve the strategy, but when using just two numbers the addition suffices.