How much differences do you see between different production lots of same ammo?
I used to have couple boxes of Federal Gold Match 308 ammo in one production lot that gave amazing accuracy from both of my target rifles.  Couple years later I picked up couple boxes of same ammo but different production let and it only gave decent accuracy.  Another is Remington hunting ammo I had purchased last year to replace ones from previous year performed terribly...again, same ammo but different production lot.  One big surprise is Winchester Super-X ammo I purchased from Walmart recently gave amazing accuracy while previous boxes I shot only gave moderate accuracy.  I went back and purchased every box they had in stock with same lot number.

I've seen pretty significant shifts in different cases or lots of .223/556.  

If I'm going to a 3 gun match that I know I'll be shooting at 4 or 500 yards I always make sure I sight in with the same case I'm going to use at the match.

couldn't have anything to do with the shooter could it?

The real fact of lot performance became realized when I shot 22LR benchrest matches. Custom rifles, scopes, one piece rest and a forest of wind devices were a fail until you found the correct lot of high end ammo, and then bought enough of that lot to get you thru the season. Makes a case for reloading when you do develop a recipe of case, powder. primer, bullet that goes under 1/2" in a particular rifle.  Regards

It's not enough to not make deaded