Understanding what makes certain cartridges more accurate (6 Dasher/GT/PPC vs 6 ARC/Creedmoor)

I have been reading a lot about cartridges as I try to convince myself that I don't need a 6mm in the least effective way possible.

Keep coming back to two questions as I read about the 6mm options:

1) Are the competition cartridges actually more accurate when evaluated on their own, or do the cartridges seem more accurate because the average gun chambered in those cartridges is more accurate because you cannot get the cartridge in a production barrel? (This question is particularly interesting considering the 6 PPC and the 6 ARC are so similar)

2) If the competition cartridges are more accurate (when all other factors are normalized), what makes them more accurate?

Bonus: If there are specific reasons the competition cartridges are more accurate, could those features be replicated in custom loading of the production cartridges?