It is likely a combination of all. You didn't mention the bullet weight. A 1:12 will stabilize up to a 168 pretty well. Anything heavier you might want a 1:10 twist. The increase in accuracy might just be the combination of harmonics, velocity, twist, bullet weight, etc that your rifle "likes", due to it's specific combination of these attributes.
This is why reloaders usually can increase the accuracy of a firearm by custom recipes in their reloaded cartridges. They can tweek any given cartridge to the point that they can find that combination that the rifle likes to shoot. I don't think anyone has figured out the algorithm needed to match a specific cartridge to a rifle configuration so you just start at 10% or so below max and add a grain or two at a time until it prints the smallest group.