The R and SR items in the boxes do not show up so often in a box unless you already cleared all the normal stuff.
So a player with normal luck will have a box full of mostly SR-item when it is close to being finished.

So normally, the platinum box gets better the more times you draw from it.
For most players it is best just to draw from platinum box until it is empty. I would newer reset platinum box until I got the platinum SR-girl at least once maybe you have to draw 294 times to get her.