Some from A, some from B. Most problems though are user error or problems. There are a certain amount of DOA items, it happens, companies allocate for that, but it's usually in the single digit percentage numbers. IF you go by comments and reviews though, the number is a lot higher.
It's part of the curse I see here even on Tom's. People say they are new to building a PC and scared to do it and rather buy a premade one. All the seasoned veterans says don't buy a premade one, build one, it's easy, watch a youtube video. They post links of components that woo and awe the posted who seems a better deal than their premade system they were going to buy.
2 weeks later you see a post by the same person saying it doesn't work, their cpu is dead, this is dead, they RMA parts, get it back, can't get it to work, etc. In reality, 90% of those are user error and they shouldn't have been talked in to building one. In 20+ years of PC work, I've never seen a dead CPU, yet I see dozens of people a week convinced their CPU is dead and RMA it.
I think people sometimes need to remember, just because you can build a PC in 15 minutes in the dark, it's not that easy. With experience comes making it easy to them. It's like changing brakes on a car. I've never done it in my life. I'm sure I can watch a youtube video and see someone do it, and if I went to a car site, everyone would say, stay after from company XXX or YYY, they'll rip you off. Just do it yourself, it's easy, watching this video......
A week later I post about the crash I had and blame the brake pad company in posts everywhere. odd's are, it was me. lol.
Problem with the internet, everyone thinks they are expert. Had 1 bad Saphire video card and all they do is bash Saphire in every post when someone asks about their cards.