Originally Posted by
Slaninar
Just a speculation, thought:
I know that knives with low hardness steel (in HRC) are hard to sharpen perfectly.
Might be similar with bearing balls. The finer the grade, the more hardness might be required to enable bearings to be machined to that specification.
Chrome steel and tool steel ball bearings have similar hardness which is unrelated to grade. Those low hardness 304 and 316 stainless steel balls the OP referenced are specialty items used only for their corrosion resistance. The low hardness is an unavoidable function of the steel's composition and, again, has nothing to do with grade.