Could be or could be the "AC-ness"?
It is. 61xxx bearings a deep grove bearings (maaaaaan) and 71xxx are angular contact bearings.
I can't say I will sleep better knowing this but I can say it's a little satisfying.
@hippy started
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It is. 61xxx bearings a deep grove bearings (maaaaaan) and 71xxx are angular contact bearings.