The explanation for this lies in the behavior of the Get Info command. When Get Info is performed on an entire volume, the statistics reported apply to all files, including invisible files. When Get Info if performed on a folder, the statistics do not include invisible files. So the number of items and space used will appear to be different although they are actually the same.
The reason for this behavior is that folders may have a custom icon. A user can give a folder a custom icon by pasting a graphic onto the folder icon in the Get Info window. Since folders do not have a resource fork, the custom icon is stored as an invisible file called "Icon" at the root level of the folder. The reason that the Get Info command does not display information about invisible files is because a user may perform the Get Info command on a folder with two items in it, if they see "3 items" in the Get Info window, this could lead them to believe that a file is missing or damaged. However, when a user does a Get Info on an entire volume, it's reasonable to assume that they want to know the total number of files on the volume and the total space used.