This is usually down to embedded content. The Reviewable items count includes these results in the user interface as a reviewer would see them, and not necessarily what the count would be if the files were being exported. The embedded files will usually include "cwembed" in their names.
To check for discrepancies; count the number of files with "cwembed" in the name that are mentioned within the XML file produced in the export, additionally count the number of errors for:
MsgUniqueErrTotal
NSFUniqueErrTotal
NSFExtractionErrTotal
LooseFilesUniqueErrTotal
AttachmentFailErrTotal