A few things that can affect point of impact:
Pressure on a non free floated barrel: Shooting from the bench with a bag or bipod positioned toward the front of the stock can cause upward pressure on a barrel and higher point of impact. From a pack on the ground pressure is spread out more evenly.
Parallax: If a scope is not adjusted for parallax at a particular distance, moving your head side to side or up and down a little can change the view through a scope. Switching from bench to prone can change the way you're looking through the scope.
Incoming wind: I always find crosswinds much easier to deal with than incoming or outgoing!
Differences in ammo. Changing brand, bullet type, or muzzle velocity can all have an effect on point of impact. (I was at the range last spring and a guy next to me was having a bear of a time sighting in a red dot on a Tavor. When he asked for some help I noticed steel cased and brass mixed together on the bench. On closer look there were different bullet weights in addition to a variety of manufacturers. Once we loaded one ammo type at a time, he noticed groups getting much better!)
At 100 yards these conditions have an effect, but it's far more noticeable the further out you go!