Live Fire vs. Ballistic App Generated Data - how much variance have you experienced?

This has been bugging me for two days after my most recent trip to the farm with my new .308

I am fairly new to shooting at these ranges (out to 700 yards so far) and it's been a fun learning experience.

I started off by using Vortex's LRBC App to generate drop charts for the factory ammo I was using.

My first 3 or 4 times out, the information was pretty darn close during live fire. It would tell me 20.43 MOA drop for 700, and i would dial it in (or hold over, i did both) and PLINK, on target all day long. This was with Federal 168g SMK BTHP. The stuff was good enough for me, except every other box the cases kept sticking when trying to open the bolt after firing.

Last weekend I decided to try some Hornady 168g A-Max. It shot wonderfully. just over 1/2" groups at 100 yards. I was pumped! So i generated my drop chart and went back to 400 and 700 yards. It showed roughly 7 MOA drop at 400. so I dialed it up... MISS. way low. (Note: we had a mild tail wind all day, so I know it wasn't a strong head wind causing the problem)

Long story short I was shooting 16" low at 400 hundred. So i had to come up significantly to get on target. Once I was on target, i was stacking shots on top of eachother on the steel. I am 99.99% sure my zero was still on, only thing I did was re-center the turret cap back to the 0 position, i'm pretty confident I didn't accidentally turn the turret.

So, after that long winded explanation my question is:

Is it normal to experience variations THAT significant from live fire compared to ballistic app generated charts? Is it possible for a particular factory load to shoot very ACCURATELY out of a rifle but the velocity to drop off dramatically compared to a different brand? A chance that 2 boxes of this stuff was under-charged?

I know there will always been some difference, especially with variations in BC and muzzle velocity out of your particular rifle. I know eventually I should tweak and create my own drop chart for my particular rifle and variables. I don't have a chronograph and I haven't been back out to investigate more. I just find it weird that the charts were dead on for one type of ammo, and significantly off for the next.

Savage 10 FCP-SR .308 20" Barrel
Vortex HS-T 6-24x50 (shooting on 18x)