Could be mechanical, if the sight shifted or your point if aim shifted. Check your components too. Did you sort brass by brand? Trim your brass all to one length? Different thickness of brass or different lengths can change the crimp pressure. Also different case volumes can affect pressure of the load and then velocity, which would result in different points if impact. You were shooting from a bench with a rest or sandbags right? You do need to take human error out if the equation as well.