A "perfectly adjusted" headset can pinch it's bearings hard enough to dent the races by a combination of factors.

Firstly, just as with hub bearings becoming orders of magnitude tighter when the quick-release compresses the axle, a quill stem expanding in the steerer effectively shortens the steerer and tightens the bearings. This often goes un-noticed as the handlebar and stem, together with lever cables, masks the symptom of slight tightness, especially with a smooth new headset in place.

While riding, the bending loads fed into the fork crown slightly tilt the crown race, creating an asymmetric "pinch" on the bearings toward the front of the headtube, which may be sufficient to dent the crown race on the first ride!
Over the course of hundreds of miles, the occasional peak load of impact may exceed the yield strength and hardness of the contact points of the crown race, denting the race.

I always leave retainers in place upon initial installation, unless the bearing count is ridiculously low (as with Steyr's 14-ball retainers, shown below).
This way, when the races have truly worn, a full complement of balls (no, not "with one removed") can be fitted, preventing the balls aligning with dents in the races.
Doing it the other way around, with an initial full complement of balls, the springy retainer (with freeplay at each ball) may not be so effective at preventing alignment with dents in the races.
Of course it also helps to rotate the crown race to a new position on the crown at this time, as well as the cup if it too is damaged. A third of a turn in opposite directions of cup and crown race might have the greatest effect to the better.

In that it causes steering problems and is destructive, a slightly over-tight headset is far worse than a slightly loose one, and unfortunately all to common.