Two answers - first - go to view and the folder setting- ensure view hidden is selected
there may be a few you had not counted in.
Secondly it is the way the advert and printed material present mega bytes and gigabytes
In the printed/desriptive material the mega is presented as 1,000 and giga as 1,000,000
Inpractice in computery - Mega is 1024 etc.
So the actual count on the drive is less than the'presented' value