I spent a lot of time thinking on this subject- and over-analyzing it. As others have said, accuracy is key. Velocity nodes are a secondary effect. It's really weird when you're doing an OCW and the velocity goes down. Ladders in the traditional sense take a lot longer. Some pundits say it's the faster way to find where to start looking for accuracy, but since you get more than one, I disagree. The good news is that OCW doesn't require a chrono, but is fine to confirm that you get both and accuracy and velocity node.
There are many factors involved, but two are key- barrel harmonics and velocity. Velocity by itself is intriguing- how a higher load has a lower velocity is tough to figure out, but it's because of flame wave propagation, charge density, charge alignment, etc. Typically the most consistent load has a full charge. That does not mean it will be accurate.
Accuracy is a black art, and I'm still not sure how rounds like Federal Gold Match work so well in so many situations.
Nevertheless, the more interesting thing to chase is the harmonic. When you pull the trigger and it goes 'BANG', a shockwave travels to the end of the barrel faster than the round does. To be fair, for non-subs, the bullet builds up speed, and will be faster than sound as it gains velocity from the pressure but it starts at zero. And- sound in metal is faster than sound in air. And^2, the acceleration is not linear and depends on a lot of factors. But, from that 'BANG' the shockwave goes to the end of the barrel, vibrates your muzzle and returns back to the receiver, reflects back and goes down the muzzle again, rinse repeat. The end of the barrel is still when just as you pull the trigger (keep your caffeine down), but will vibrate several times as the shockwave goes down and back, pausing in between cycles. The most consistency is when the muzzle is still, so the goal is to find a velocity that the bullet exits when it is still. Look at Blake's groups above- they grow (vibrate) and contract between different charges. Nirvana is when you get both a non-vibrate and a consistent velocity for the best accuracy.