A ZIP files should count as 1 (one) file if you had them in the disc. So, it would only make a dif for to determine your supplier counting criteria.
If yoy have a multisession and you see much less files than the necessary to total the occupied space, it's propbably because Explorer is not "seeing" all of them.
Multisession carries this kind of problems quite often.
Did you try to open a project in Nero (or other burn that supports multisession), put your disc in the drive as "source" and see how many files are there?
(just a suggestion...)