As far as I understand it, it’s a -10/+10 chance on each item in the box based on the highest hero power item you have seen on any character. In example, if you’ve seen a 205, your next box has 195-215 range on items you could possibly pull out of it.
So each box you open assuming they are champ boxes, you increase your ‘secret number’ slightly. This means the only way to increase is very slowly by opening as many boxes as possible.