I just read a post on the forums page where a woman said she was offering a VERY wishlisted book on the forum instead of posting it in the system... I don't understand what the benefit of that would be. I know for a fact that that book has a quite a few members wishing for it. So it's not like the book wouldn't have gone fast.. so why not just post it in the system?

Probably because she wanted to have it picked up quickly.  Sometimes books can go through several people before they get picked up especially if the WL is really long.  Someone recently posted (it's probably still on the 1st page) about posting Eat, Pray Love and it going through around 20 people before it finally got accepted.  This is really annoying especially when it just times out.  It's not so annoying when people hit decline pretty quickly. But I posted a WL recenlty that timed out 5 people and was turned down by one.  So it sat for a week before it got picked up. 

I think the thread title if Eating Through the Eat, Pray, Love WL-check it out.

It might be unpostable, too.  I ended up giving a copy of Eat, Pray, Love FREE through the bazaar because I tried to post it then realized it had a slight spot of water damage in one corner.  I offered it as free with purchase but the book someone ordered off my shelf was one that didn't "take" when I manually removed it the week prior (be sure to double check your bookshelf when you delete things manually!!)... so I ultimately just told her never mind, returned her credit, and mailed her the book for nothing.  :)  She might be one of the 22 people who forgot to remove it from her wishlist, in fact!!  ;)

Also, in some topic forums, members tend to build little reading groups and buddy lists, and sometimes they try to share highly desired books with other members of those subject forums first because, as Mary stated, they want the book to be picked up quickly or go to someone who will enjoy it.

I'm not arguing for or against the FIFO process here, just offering my own observations.

Cheers,

Catt