Posted final scores are calculated without scores hidden from students. Unposted final scores are calculated with scores hidden from students, hence they more accurately indicate where students stand in the course. To quote the guide about exporting the gradebook to CSV:
Current score reflects the total while ignoring unsubmitted assignments, and the final score counts unsubmitted assignments as zero. Assignments with hidden scores are unposted grades and shown in separate columns for unposted current score and unposted final score, respectively.
