You are not showing the location of publication, however in the U.S., while Copyright is established the moment the work is created, the only effective way to claim infringement is to have registered the work with the U.S. Government's Copyright Office (http://www.copyright.gov). The date shown is more than likely the date that they received confirmation of the copyright registration from the office.
Subsequent editions will also refile copyright, so you can have various dates or a range of dates shown. On Molecular Biology of The Cell by Alberts, et.al, the copyright page has the 2015 copyright for the sixth edition but also lists the 2008 date for the preceding fifth edition.
I have generally seen date ranged copyrights on software, so if you select About This Mac on an Apple Mac computer, the splash screen shows (c) 1983 - 2015.
Even though copyright exists from the moment of creation of the work, an important reason for official registration has to do with the damages that you can seek for infringement. The following is from the Copyright Office's FAQ
Why should I register my work if copyright protection is automatic?
Registration is recommended for a number of reasons. Many choose to register their works because they wish to have the facts of their copyright on the public record and have a certificate of registration. Registered works may be eligible for statutory damages and attorney's fees in successful litigation. Finally, if registration occurs within five years of publication, it is considered prima facie evidence in a court of law. See Circular 1, Copyright Basics, section “Copyright Registration” and Circular 38b, Highlights of Copyright Amendments Contained in the Uruguay Round Agreements Act (URAA), on non-U.S. works.