The box they are talking about is the box that comes from the factory. By mixing all of the flavours you only need the shelf space of one box. They place more

The box they are talking about is the box that comes from the factory. By mixing all of the flavours you only need the shelf space of one box. They place more boxes behind that one. If they needed one for each flavour, then they would need three boxes, side by side taking up three times as much space.