Well, it can't hit zero because you'd need equal numbers for that, and they're not coprime ;)

It can't be negative because you cheat and always put "larger - smaller". See your own example: you go:

    13 - 7 = 6 -> 7 - 6 = 1
     a - b = c -> b - c = d

But then then you do:

    6 - 1 = 5 -> 5 - 1 = 4
    a - b = c -> c - b = d

Which switches the order. What if you don't?