You're mistaking a frequency-related term for a quality-related term.
'Rare' doesn't mean 'better than uncommon'. It means 'encountered less frequently than uncommon'. A 'like-new' Ford Edsel is 'rare'. A 'like-new' 2018 Ford Explorer is common. The Explorer is better than the Edsel in every possible metric, except 'sale value to a collector'.
From a mechanics perspective? The fact that the armor doesn't need attunement is also a big difference once your characters start accumulating a significant number of items.
