You don't know exactly which book you need until the first day of class (there's "new editions" all the time), you'll need the book the first week of class, and you won't get it for a week and a half ordering online unless you do the expedited shipping which at that point you might not save anything.
I had that exact situation happen to me with at least 1 class every semester. It's happening more and more too. The professors get pressured by the schools to upgrade to the new editions if they're resistant to change. Some of them co-authored the book and every revision so they're making money from it changing every year. I compared editions of a book one time and it was such a drastic change I thought I was looking at 2 different books on the same subject.
It's all a money making racket.