ACSI sites change in reference numbers

I have just been looking at the new 2013 ACSI books and transferring visited and recommended sites from my 2012 books.  The reference numbers of sites have changed so I have to look up the Place Name Index to cross-reference.  Do they do this every year? You would think that they would leave gaps in the numbering system to take care of new sites added (200 approx. this year)!