I think what MG is saying is that this bearing was badly made.
He says that the remaining balls are all round but that 4 balls were missing.
Now if those balls had disintegrated then there would be gritty grease in there and the remaining balls would be scored and pitted.
What in fact happened was poor assembly of the bearing with 4 balls missing.
Chances are the rest of the balls were evenly distributed so it ran for a while.
Then, under stress (heat, load) the cage collapsed into the gaps where the missing balls should have been and that locked the bearing, spinning the axle and causing the damage.
Moral of the story is when changing your bearings, pop one of the dust seals off and check that yer balls are where they should be.