The thing about chances is that as long as it's in the realm of the possible, if the system is open ended, you wait long enough and it will eventually come to pass. You may list the "hit" rate as 20% but, theoretically you could open a trillion, trillion, trillion Mystical Objects and not get a single diamond payout. Statistics are only useful to predict the distribution of an aggregate set, or to discover the statistical possibilities in a given set. They aren't usually very good at predicting a small set or an individuals experience. If you try to apply a "hit rate" to your own case you will find that sometimes you get the sense that the system is "rigged," and not in your favor.
I open about 2-3 Mystical Objects a week. If I do 2 a week and 1 offers a 20% chance of getting the diamonds about 10% of the time I open Mystical Objects should get diamonds. That's what the odds predict. But since I'm only one small set, there is a pretty good chance my numbers won't match the aggregate. In fact, about fifty percent of the players will be under the predicted rate and fifty percent will be above it. The fifty percent above will not notice it too much (except at the extremes) a they will accept the positive as "par for the course" unless they are 80 to 100 percent above the norm. Then they will just think of themselves as "very lucky." The ones below the average, though, will start looking for explanations when they get 10-20% below the average and will become more and more convinced that "something is wrong" if they get to the 80-100% range. In other words, humans are perfectly fine with "good luck" as an explanation, but there can't possibly be as much "bad luck" as I'm getting. The father they go down from the "norm" experience the more they become convinced somebody is manipulating something.
It's interesting to note that Los Vegas runs in the same way. Get to lucky and they don't believe you can possibly be that lucky. So they ask you to leave. Get that unlucky and well, it's just an unlucky day, right? In other words, because your "luck" is bad for them, it must be something other than luck. Your "bad luck" is to their benefit, so it's just "the luck of the draw" and they never ask you to leave.
AJ