Lots of things...
Shooter Error:
- Shoulder pressure
- Cheek pressure
- Rear bag settling
- Stock catching on something - rear bag, front bipod, sling stud, etc.
Reloading Error:
- Inconsistent powder charge
- Inconsistent priming
- Inconsistent neck tension
Environmental influences:
- Wind direction*
- Terrain features**
- Mirage***
* By itself, wind direction may or may not show an appreciable amount of influence on vertical impact - some swear by it, some at it.
** Terrain features tend to magnify the effect of wind direction - in effect, 'funneling' or concentrating the wind in a manner that may push the shots up or down.
*** This one can be both environmental (mirage increasing as the day warms up, heating the ground) and shooter error, from not using a mirage band, especially on warm/hot days with little to no breeze, allowing barrel mirage to adversely effect your sight picture.
There's probably any number of other factors, but those are the ones that come to mind right now...
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