If you put in two identical bowls, and one doesn't have a rocking seat and one does, and you changed the seat and it still rocks, and you can identify differences between the two bowls that make the second bowl not work as well and not fit the seat, then...yes, it's defective.
That said, whether it's defective "enough" for the manufacturer to take it back and replace it is the real question. It passed their quality control, presumably.
However, some companies (AS being the one we discuss most on here) seem to have a policy where they draw the line as to what's "acceptable" at a pretty-unacceptable place, and seem to figure that they will try to sell the thing and hope that it isn't returned by the customer. But the retailers DO take them back when the customer makes the effort to rip the thing back out and cart it back to the store.
So, don't "make do" for the next 15 years with something that isn't right. I would insist that they replace it with one that works as well as the one that you like. And if you have to make a few efforts to get them to do that, I would do so.
As has been mentioned on this forum repeatedly, a molded vitreous china product is a product that requires lots of care, effort, monitoring and tweaking to produce consistently. It really is like baking a cake -- getting those Twinkies to come out consistently is a real challenge.
One reason that Toto gets such high marks on this forum for quality is the effort that they make to produce a consistent product, their willingness to throw away imperfect castings rather than trying to sell them, and their insistence on taking things back and replacing them on the rare occasion that one is not right.