Well here's some possible reasons:

You were not #1 when the book was offered to you: The 1st request could have been turned down for RCS or been on vacation hold. Someone could have posted the book directly to you bypassing FIFO.  As for the other one-you should have gone to #1.  Maybe someone had this book on vacation hold. It was posted and you ordered the book. Meanwhile the other person came off vacation hold to resume the #1 spot and 2 other people added it to their WL-probably after seeing it in the Books Recently Posted list. 

If your account is on hold and you are the only wisher for a book the book becomes available to other members. I had my account on hold for several months because of a cross country move.  A few of my WL books were not longer WL books when I came back.

At the time when you marked the book  RWAP, if the person behind you (who was at the front of the line now) was offered the next posted copy and had not responded to the offer yet. That person would stay at the front of the line until either the transaction is agreed upon, they decline the offer or the 48 hour limit is reached and the transaction cancels.

Odds are someone ahead of you went on a vacation hold which removes them from the wish list and then returns them to their place in line when they come off the hold.

Hope you get your WL books soon!!