How do you even distinguish between grade 100, grade 300 and grade 25 bearings on a bicycle? We're not talking wind turbines here. All things being equal, it would seem (and I'm no engineer) that other factors on the bicycle would effect the smoothness of the assembly. Like manufacturing defects on the cups & cones of loose bearing parts. In all these years I've never been able to tell the difference except when comparing a very cheap wheel (< $50 USD) to a high end racing wheel ($1000 USD+). Or something like a Chris King headset vs. department store headset.

What am I missing?