Why are little pieces of cardboard lighter than a whole piece of cardboard?

This is because little pieces of cardboard is the same thing as splitting the weight, so splitting the weight of it means the total of all the pieces won't be exactly the same as a whole cardboard.

I use small cardboard boxes for organizing drawers, etc,---also break them down and use it in the garden to kill weeds, and I add some to my compost ... side to one edge that's somewhat shorter than the magazine, to enable one to remove one magazine without removing the whole box from the shelf.