It is called the law of large numbers. Basically if you have a big enough pool of sales you will see every probable scenario.
In addition to a certain percentage of defects, a percentage of people returned them for a sudden shift in their financial situation, a percentage of them have been returned because the owner liked one of the previous models better, a percentage of them have already been accidentally damaged, a percentage of them have been lost or stolen, unfortunately percentage of them no longer have owners because they died, and that is just naming a few of the possible outcomes with 3.5+ million sold.
IMO, worrying about defects is about the same as worrying they are cursed because some people have had something bad happen to them after buying one. Wait a minute, my house was partially destroyed because of a tornado... got to call a priest...