HELP! Old Posts Dropping Into Abyss / Expiring Then Coming Back !?!

A moderator told me that she was doing some cleanup by removing "moved" redirects from our main forum / category.    There are 26 pages of topics in the main forum and she deleted about 25 redirects.   She expected the page number to drop from 26 pages to 24 or 25 pages of topics, but it remained at 26... well that was weird.

I restored a backup of my forum in a new folder and started out to duplicated exactly what she did.   

First:  I noticed the last post on the last page (page 26) was on 02/04/2007 4:14 pm.

Second: I deleted 10 moved redirects and (this is where it gets weird) the last post on the last page (still page 26) is 02/01/2007 5:48

What's going on?   It looks like posts past page 26 are falling off / being pruned, but once I "make room" by removing other topics the old topics re-appear.

I haven't pruned anything and there aren't any settings (that I can find) that would hide topics past a certain date / page / number.

Any ideas what the heck is going on?