Error bars go in both directions so better than expected performance is not unusual at all. "Better than expected" doesn't mean it's doing something it shouldn't have been able to do. It just means that variability stacked up in a favorable way. You model this stuff to account for uncertainty in all your components so the expected performance already assumes it's not going to be perfect. Frankly, in a complex system like this, it's difficult to even quantify what "perfect" even is because there are infinite dispersions and it's not always obvious what combination of variables is best.