Why is size on disk a different (larger) size then "size" itself, please see attachment

Why is size on disk a different (larger) size then "size" itself, please see attachment

I have copied a NAS fully from an old NAS to a new NAS, one on one, no errors. As you can see, the "size" is virtually the same on old and new.

Left side is old (source) and right side is new (target)
Right side is a bit larger since people already put new files on it.

Language is Dutch, so:

grootte = size
grootte op schijf = size on disk
bestanden = files
mappen = folders

I would expect all to be the exact same size and count.