If the friend's groups shifted in the same direction, the conditions changed like 4xforfun stated. This is why many F-Class shooters try to get their 20 shot string off in short order once they get dialed in.
Wind, Sun, Temp/Pressure
Other than a Barret BORS system, I don't think a custom rifle
solves any of those issues. And, it has it's limitations also.
An umbrella over the chrony and keeping your ammo at a relatively constant temp might help eliminate a couple of variables.
Wind can be deceptive including up/down drafts around trees/obstacles and it can be very deceptive shooting across hills/ravenes as compared to flat ground.