Gremlins I call them. Nasty little beasties that infect all things with moving parts. The more complex the system, the better the chance for one of these little buggers to gnaw on something and jam a proverbial gear. A modern rifle, optics, and ammunition is a rather complex system of chemical and kinetic reactions, and there are many habitats for gremlins. Best practices towards consistency of the system can mitigate their damage, but they are still there waiting for a lapse in QC, good old fashioned entropy, or a seemingly insignificant change to give them succor.
Many shooters read too many online articles, forums and old fashioned paper rags on extreme accuracy. Many shooters sugar coat their grouping capabilities on said medias. This leads many shooters to allow the pursuit of perfection to be the enemy of the sufficient.
I shoot NRA high power, CMP matches and I hunt big game. I would be ecstatic to have a rifle consistently group under 1" at 100 yards across 10 rounds. In the real world, with service rifles or hunting rifles, about 1- 1.5MOA range is the best I've seen reproducible. I consider smaller groups in smaller numbers of shots to be outliers, and useful to narrow down options for further testing, nothing more. Over the years, I have found loads for most of my rifles that will maintain 2MOA or better across a range of atmospheric conditions and will usually hold this standard with a lot change in a component. This is good enough for my purposes, and since I also like to have money in my bank account and do things other than load and test ammo, this is where I stop.
Cloverleafs are fun, but I don't chase them into black holes.
Here's a real world example. Not my target or rifle, but submitted in the match I'm hosting as an unofficial "look at this". This is a tuned BR or F-class rifle. It is BUILT. Target courtesy of Nature Boy. 10 shots in aprox .9 MOA at 200 yards. This is about as good as it gets. If you're chasing a higher standard consistently, you better have the money, time, gear, and money to chase it (money intentionally mentioned twice).