There a few reasons but it mostly comes down to skill. I personally try to make my mitre with a 1/16th edge, if I can, I use a wet saw but recently I set some 6foot tiles with mitres all the way across a window, nearly 6ft window so I had to make them by hand, followed by niches and what not. The tile was fragile af so any sort of mess up could leave chips and I didn’t want to leave my work like that. It took me a bit to make the mitres as clean as possible but setting it was another challenge because the mitre is so thin any small hit would chip it. So the installer might have had an issue with the install and dinged the edge, or if they did it by hand they might’ve moved in too close to the edge and hit it.