Author's ego probably. Or just an attempt to get people who've already read it to buy a second copy (the version in question was released 10 years later.)

> A special tenth anniversary edition, which includes the "author's preferred text" and 12,000 additional words, was published in June 2011 by William Morrow. The tenth anniversary text is identical to the signed and numbered limited edition released in 2003 by Hill House Publishers, and to the edition from Headline, Gaiman's publisher in the UK since 2005.[2] The tenth anniversary edition marks the first time the author's preferred text has been available in wide release outside the UK.[14]