If it's a win / lose outcome and both win and lose have a probability of 1/100, I'll make 99x my stake by betting on both win and lose at the same time.

If both outcomes have a probability of 0.999 (summing to almost 2), I'll barely make any money if I'm right, and lose my money if I'm wrong.

So when probabilities sum to less than 1, it's good for the gambler, and when they sum to more than 1 it's bad for them (and good for the bookkeeper).