With things that are mass-manufactured, yes, there will be some defects that get out there and then are returned. That's a fact of "life" in the manufacturing world.
And then, in the early part of its "life", an new product will typically start out with more defects. But the "makers" take those defects seriously, working to identify any "class" problems, defects that are caused by a problem in the actual manufacturing process, as opposed to the incidental defect that has no "root cause" in the mfg process.
The ideal goal, of course, would be to have 0% defects, but that is not a realistically attainable goal. Under 5% is a definite goal.
And, Quality Control as a comprehensive final test will likely only be performed on a few off the assembly line, as opposed to little "mini-tests" that are likely routinely done on each unit in the line.
I don't know Canon's system, but I did work in printer manufacturing for a bunch of years, and so have an idea of what things are likely like with a Canon plant.
So, with the 7D, I'd imagine that in the early days things came up, cameras got returned, they did whatever they could to improve the product and process, and so now, occasionally a camera will get shipped that has a defect and has to be returned, because that's "normal".
It was that way with, gosh, maybe every new release, especially with ones that had significant re-designs. I particularly recally the 5D Classic and the 1DMk3, because I own both of them, and then the 5DM2 had issues early on (and still does have certain things) that needed addressing.
What happens then is that we hear about that occasional defective unit and a little bell rings. Here in POTN, we have, well, a whole bunch of people, of whom I can imagine that thousands are 7D users, although I've never seen any way to determine the actual number. But, if say 10 out of 1000 7D users had some type of issue, we'd hear about it loud and clear, but that would only be 1% which, believe it or not, is within the "tolerance" range for defects, although like I said, I don't know the particulars for Canon manufacturing. Maybe it's 1 of 1000!