Parallax won't change bullet impact 1/4" at 100 yards. That ain't the problem. From Leupold owners manual.
Quote:
m parallax occurs when your eye is
at the very edge of the exit pupil (Even in this unlikely event, our
4x hunting scope focused for 150 yards has a maximum error of
only 8/10ths of an inch at 500 yards). At short distances, effects
of parallax do not affect accuracy (using the same 4x scope at 100
yards, the maximum error is less than 2/10ths of an inch )
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I also zero at 100 yards. The scope is going to be about 1" above the bore so at ranges measured in single digit feet the bullet may impact as much as 1" below the POA. It depends on the exact cartridge and load, but a 100 yard zero will keep the bullet no more than 1" above, or 1" below POA from the muzzle out to 130-150 yards. With most loads around 2" low at 200 yards and 10-12" low at 300 yards. This is pretty much the same with any modern cartridge shooting pointed bullets at 2600 fps-3000 fps. You don't start seeing any real difference until after 300 yards.
I don't like to zero so that the bullets are ever hitting high at any point. That just complicates things at close range and does nothing to help at long range.
Shooting UP or DOWN may cause the bullet to hit a little HIGHER than normal. This is more of a problem with archery than rifles. You'd have to be a LOT higher or lower than the target and the distance would have to be several hundred yards for it to have any real impact.
Good illustrations here explaining what happens shooting up or down. I'm using the top left graphic. If you're elevated 20 yards above the ground it may be 30 yards from where you are located along that leg of the triangle. But in reality the projectile only travels 21 horizontal yards to the animal. That is enough to be an issue with archery gear, not a rifle.
Multiply those numbers by 10 and now you have a situation where it would matter. The range finder may say 300 yards, but in reality it is only 210 yards to the animal. That error would cause you to hit high. It would be the same if reversed and you were shooting uphill. The horizontal distance to the target is closer than it appears.
https://www.google.com/search?q=shoo...w=1242&bih=597
I think you are simply missing low because you're holding the rifle in a non-conventional position shooting at a downward angle.