Point of impact shifts between serials?

Down at the range today doing some sighting in.

6 targets. 5 shot group on each target.

First 3 targets the verticle main of impact didn't change.

However, targets 4 and 5 had a significant change in main point of impact in terms of verticle. It was about 3 inches low. All the groups were still tight.

And then target 6 was the same as the first 3 targets.

The air temperature would have come up no more than 2 degrees during the course of the firings. The gun was cooled before moving between targets. Shooting techniques remained the same. Same ammunition batch.

What could this be??

Keep your ammo out of the sun and cool as possible. As the day warms up your speeds will increase causing point of aim shift.

Same effect is caused by leaving a round chambered too long in a hot barrel when shooting a string

Ammunition with temperature change - this I would have thought would more likely show a more continuous change in POI. What you have described is two well defined vertical impact positions. This would indicate to me the rifle and associated equipment - or the shooting setup - moving between one of two states.

If you are prepared to swear to the consistency of your shooting & setup throughout the six groups to be able to remove that from the possibilities, then one of the usual culprits is probably the cause: movement in the bases or rings, internal movement of the scope reticle, unintentional but well defined differences in the ammo, (different loads, bullets, seating etc.), bedding - moving between two different metalwork-to-stock positions.