90% sure it is just spec level, Ultegra/XT v Dura Ace/XTR
Yep, supposedly the 91 has 'better' bearings, but how and to what extent they're better I don't think anyone knows. Except maybe Mr Shimano.
@Brommers started
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90% sure it is just spec level, Ultegra/XT v Dura Ace/XTR