Where? I don't see it anywhere. There's some discussion of total volume perhaps being non-independent from box to box, so an underfilled box is followed by an overfilled one, but this isn't the point the grandparent is making. The grandparent is making the point that there are probably systems in place to prevent a box from being 80% red, say, so that the assumption each individual skittle in a box is independently uniformly drawn from each possible color does not likely accurately model the dynamics.