A more accurate statement would be that in the world of AR15s the same people that make Part A for Brand X, also make Part A for Brand Y; but to a different standard and tolerance - this is why Brand Y can sell it cheaper.
Another thing that happens a lot is Brand Y buys the Part A that Brand X rejected because Brand Y has lower standards and most of the time it doesn't make a difference. This is particularly prevalent where the standard has some military purpose (like being able to function in -60F weather) that 99% of Brand Y shooters will never know about or use.
And that is fine; because the whole reason Brand Y exists is that not everybody needs what Brand X offers, so why pay extra for features you don't need? Just don't think you are getting Brand X at Brand Y prices.
But that's exactly what's happening. It's like buying LEDs. The supposedly higher quality ones come out of the same bin as the rest of them, the only difference is you're paying someone to sort through them for you and say "I think this one is marginally better" based on whatever arbitrary criteria. In the case of LEDs it's normally "Well I think this one looks better when I test it so it costs more..."
Pick any part of the AR15, let's say the trigger. Let's take a box of 1000 triggers from the plant and dump them out on your desk. There is someone out there who feels they must absolutely have the best trigger out of those 1000. You can make money by playing on their demand. Now maybe they have a good reason for that or maybe they don't, it doesn't matter, you're here to make some money.
But you've got to make it sound good to the buyer, so you say "Well you see this particular batch of five triggers, I just did a test and I can guarantee you they all have a minimum Rockwell hardness of 58." You talk it up somehow.
It's nothing dishonest, that person was willing to pay you to say that 5 out of 1000 should cost more for some arbitrary reason. The buyer is not paying for a better quality part, the buyer is paying for scrutiny and reassurance.
The truth is in reality, those triggers all came from the same assembly line on the same tooling made by the same people. While there is no guarantee the rest of them meet that same spec, the likelihood they do not is quite poor.