Imagine a tape that has only 3 files on it. All three were backed up 30 days ago. One file has since been deleted from the server. One file was changed two weeks ago and so an updated version was saved to another tape that is less than 30 days old. This leaves one file that is now 30 days old, has not changed and therefore has not been backed up again and still exists on the server.
If we want to be able to restore an entire file system as it exists today, then we cannot delete this tape. We would lose the only backup copy of that third files. 
Presstore therefore looks at file #1 and says 'OK, this file no longer exists on the server. I don't need to keep this backup copy. If it were just this file, I could recycle this tape.
Presstore then looks as file #2 and says 'OK, this file has since been resaved to a different tape. Therefore we have another backup copy and I could recycle this tape.
Then Presstore looks at file #3 and says 'OK, this file still exists on the server. This is the only backup copy. Therefore I'll need to resave it to a new tape before I can release this old tape.

Of course, normally you would have a lot of files like file #3 and fewer files like files #2 & #3. Therefore, what looks like another full backup is Presstore resaving those files that are 30 days old and still exist on the server. 

For a video on this topic, please see here: http://p5masterclass.jpy.com/video/98444759

The discussion of progressive backup starts at 8:17 mins into the video.