So synsuck goes around to all the syndicated account feed sources and "do we have new content yet?"-s at them. (Related: if synsuck asks this *too* often, some websites, like slashdot, will so pull this thing over and smack it.) When there is new content that synsuck hasn't seen before, synsuck grabs it and runs back with it to the feed journal and stashes all those cool new entries there. Then your friendspage reads the syndicated journal, and goes "Ooo, shiny!" and posts stuff to your friends page.
However, syndicated accounts do not store their entries forever. After two weeks, any given syndicated entry will go POOF! off the face of the planet. Imagine Momma Syn coming through and picking up very used facial tissue up off the floor and throwing it away. (This is more true in some syndicated comment pages than others. Roll with the breaking-down scary analogy here.)
Ordinarily, everything is all good and all right. But, imagine what happens if the source of the syndicated feed stops updating for a while? Picture it. Over two weeks without posting. What a long time! Syn has no attention span. None whatsoever. Not that long. (Now picture a very happy retriever, very eager to please, very literal about following directions, not so smart.) All the entries in the feed journal just go away. Woops. So when synsuck goes out and the feed has *finally* updated, it goes, "ZOMG AN UPDATE!!" and immediately checks back with the syndicated journal on LJ to see how much it needs to catch up on.
Now remember, it's been two weeks or more. All the previously-posted content is gone, gone, gone. Syn has no attention span. So synsuck sees ZOMG A BLANK JOURNAL!! So since if it were old, synsuck would see a copy of it in the journal, all the stuff that synsuck sees on the feed source must be NEW!! And what do we do with new content suddenly being poured into a journal? Why, record it all, of course! Thirty fresh articles, all shoved into the syndicated account, and straight to your friendspage! Because syn is not bright enough to figure "Um, I had this before, didn't I." Those entries are only temporarily stored, and therefore don't exist anymore.
...It's a bug.
What they're doing to fix it will depend on implementation and things like actually being practically able to figure things out. But it is a Known Thing, and that's how it does it.