Also i think lots of people bought the first three core 4e books(i know i did). But a large chunk ended up not liking it and playing just a few sessions. So it wouldn't surprise me if they did pretty okay out of the gate only to hit a steep decline soon after.
There were several people on ENWorld (and, if I recall correctly, WOTC's forums) that stated they were preordering 4,5 or even 6 copies for their group long before there were any previews of the actual rules. As you note many of them later regretted their purchase and sold them after reading the book and giving the game a try, There were also several of those people that had bought multiple couples, who found that the people for whom they bought those extra copies had no interest in trying 4e.
The funny thing is that we are already seeing people stating that they are sold on 5e when the game is not even finished.