Rather than counting error it’s likely the ~1 ton weight of stacking bills like this would deform the lower sections and possibly stress the glass depending on

Rather than counting error it’s likely the ~1 ton weight of stacking bills like this would deform the lower sections and possibly stress the glass depending on thickness. So rather than random filler there may be internal structural bracing so the outside of the cube looks nice and neat. Simplest way to double check is if top and bottom corners have the same bill density.