They explain in detail here, but it seems the methane on saturn is absorbing and not reflecting, leaving it darker.
https://webbtelescope.org/contents/early-highlights/saturn's-rings-shine-in-webb's-observations-of-ringed-planet
https://webbtelescope.org/contents/early-highlights/saturn's-rings-shine-in-webb's-observations-of-ringed-planet
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