This has to do with your vestibular sense and is applicable when you setup your scope/reticle to your "natural hold", not perfectly level with the action/rifle. When you bring the rifle into your shoulder it will not be square, it will be canted based on your build, if you have a stock that can adjust for cant then you can set that up so the rifle is square or perpendicular to plumb but for most without adjustable cant your rifle will be canted with your natural hold which means if you mount your scope so it sits in your rings level to your action (via the Spuhr tool or using feeler gauges, etc.) then each time you bring your rifle to your natural hold and your vistibular sense takes over you will cant the rifle which means you will look at your bubble and see it is "off" and have to adjust each and every time; however, if you mount your scope so the reticle is plumb with your natural hold then each time you bring your rifle to your shoulder you should be close and have to make very small adjustments to the cant of the rifle in order for that reticle to be plumb.
Hopefully that makes sense, I always find it difficult to explain in writing, Thomas does a decent job with the video but I wish there was a better video that explained this more appropriately. Frank, if you feel so inclined I think if you did a video it would help the Hide members understand this better as I still think many are mounting their scopes level to their action without cant adjustment on their stocks.
My first thought, and i could be mistaken here, is that if you are canting the rifle due to your vestibular sense, but have leveled the reticle to a true level, so that the erectors elevation travel is colinear with vector of the earths gravity, then your rifles bore is now offset laterally from your scopes center, relative to gravity. This would introduce some horizontal error proportional to the scopes height over bore, and proportional to the magnitude of the cant/offset.
Similar to the old russian side mounted scopes.
Just to make sure im understanding, ill try and describe what im visualizing.
If behind the rifle, looking at the rear face of the buttpad, and due to my vestibular sense i have the rifle canted clockwise 1°.
Effectively, my scope tube is no longer directly above my bore, but is now both above and a little offset to the right of my bore. But, knowing this 1° cant would occur, ive leveled my reticle so that the scoped elevation travel is still true-vertical relative to gravity.
Now when i zero the rifle @100 yards, my bullets flight path has an elevation component (so as to intersect my line-of-sight @100 yards) but also now has a small horizontal component, (traveling somewhat to the right to untersect line-of-sight @ 100 yards).
It would also be the case that beyond 100 yds, the horizontal component would still be present and the bullets flight path would continue to diverge from my line-of-sight, with the divergence proportional to the distance?
Is this amount accounted for somehow, or is it not significant enough to be of concern?