A couple of thoughts-
First, if accuracy dispersion is in both planes, why do we worry about vertical dispersion only and not horizontal dispersion? The reason is that as distance increases, accuracy dispersion becomes a very minor factor with modern, accurate rifles. Gravity and wind are the two major factors that we have to deal with, and wind is so variable that accuracy dispersion becomes immaterial, and is lost in the resolution noise of the wind. Not to mention that after a node is found, minor adjustments can be made to shrink groups and reduce horizontal (and vertical) dispersion. Gravity, however, is consistent, and its effect on projectiles with different muzzle velocities grows disproportionately as distance increases. So we really want to find a way to minimize the POI variation induced by gravity and minor MV differences. To illustrate, if we find a node and work up a load in that node that shoots 0.5 MOA on average, at 1000 yards the mechanical accuracy dispersion of that load is 5.24". So there is a variation of 5.24" of vertical dispersion inherent in the load itself. Now, if we used a 0.625 G1 BC, 3000 fps load with a MV extreme spread of 40 fps, gravity would induce a variation of roughly 11", and that numbers gets bigger the farther out you go. So if we can time the bullets exiting the muzzle so the slower bullets leave when the muzzle is pointing slightly higher, and the faster ones leave when the muzzle is pointed lower, we can largely cancel out that gravity-induced dispersion.
Wind, on the other hand, is unpredictable, variable in speed and direction, and wind drift at 1000 yards dwarfs the 5.24" of mechanical accuracy dispersion. Using our 3000 fps 0.625 BC example, even a 10mph wind can induce 70"+ of deflection.
As to the suppressor accuracy observation, I can think of a couple of reasons to explain increased accuracy when using a suppressor. The first, is that one factor in accuracy is muzzle blast causing bullet upset as the bullet leaves the muzzle. A perfect crown and a well-balanced and stabilized bullet help mitigate the muzzle blast causing instability as it squeezes passed the heel of the bullet. A suppressor reduces this high-pressure force around the bullet as gas escapes, which minimizes bullet upset as it leaves the muzzle. Secondly, it's important to note that finding a node doesn't eliminate vertical dispersion entirely, but it minimizes it within the limits of the "whipping" motion of the barrel. A suppressor does dampen the pressure wave emanating up and down the barrel when ignition occurs. If we have bullets leaving when the muzzle is positioned differently, causing accuracy dispersion, that dispersion is minimized by reducing the positional variation as the pressure wave goes through the barrel. If we magnify the effect of the pressure wave for a minute, imagine that the barrel whips in a figure-8 motion as the pressure wave goes through it back and forth, and that the extreme bottom and extreme top positions differ by 2MOA. That means that in a worst case scenario, if a bullet exited the top of the muzzle's path and another exited at the bottom, the group at 100 yards would be 2". If we hang a weight off the end of the barrel, dampening the harmonic motion so that the positional extremes of the figure-8 only differ by 1MOA, then the group would be reduced to 1".
So, while finding a node and selecting a group based off of a ladder test minimizes the effects of MV variation on a bullet's trajectory at range, reducing the motion itself means inherently smaller groups.
