I really don't know the "magic" that happens when a box is stored in the Unclaimed Box tab.
But I think the randomization/chance of getting an item is triggered when you open the box.
So... would you get the same item if you open a box at different times?
You may or you may not.
The probability of opening the box and getting the same item is still there, I think.
Unless---- the randomization is triggered upon receipt of the box. Therefore, if that is the case, no matter when you open the box, you would get the same item.
I don't really know at this point.
Interesting idea, though. And... since the developers would not even disclose the drop rates, I doubt any information on this matter would even be shared to us. Got a time machine, maybe?