I have seen people shoot a 3 shot group of only a 1/4 inch and brag about it but if they shoot another group it may not hit the same spot exactly or it may be a bigger group. Eventually after so many 1/4 inch and 1/2 inch groups hitting here and there you eventually end up with a total group of about 1 and a half to 2 inches.
barrel warpage from long shot strings can do these things. Even with the heaviest barrel and waiting several minutes between shots there is still a rising heat level that will change the point of impact slightly.
Even the .220 swift at 100 yards with a 10 mph crosswind will drift at least 1/2 inch if not more.
The temperature may be different than it was when you zeroed the gun.
group size is one thing but putting them in the same spot day in and day out under differing conditions is pretty difficult.
Try the penny challenge at 100 yards. tape a penny on a target and try to hit it. You might find you and your gun are not the moa all day combo you think you are.
Not to say there arent people who might be able to do this consistently but almost certianly most cannot.