You are confusing 2 things. As you open more boxes, the odds of one of them not being a premium box gets smaller, but that is only because you are taking a random number in a discrete, uniformly distributed set of integers. The problem is: The only 100% guarantee you'll get is by opening a infinite number of boxes. If you toss a coin 10000 times, it's veeeeeeery likely you'll get tails at least once, but it's not 100%. I got 2 premium boxes in a row on my first and second tries, but you might never get one in your lifetime.