It is because no box among the 10 contained 100%. The highest I have seen is 80% from a picture posted by someone on facebook. If there were 10 boxes, each box with a different % like 10, 20, 30 , 40... 100%, then it would be another thing. Another trick is that the prices are not fixed (auctions) and which will likely inflate like you yourself observed.
Then there is something else: what if the user would get the same % no matter what box he would choose, that is it would not be a matter of which box was chosen?