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Happy marriage through compromise.
(feddit.uk)
My wife married into my Warhammer collection. We have a Warhammer room. I'm pretty sure she doesn't even like Warhammer but has her own painted figurines.
There is a possibility that his preference for peanut butter isn't that strong, and it's easier to just stock one kind. It's also possible she eats significantly more peanut butter than him, so his jar would sit unfinished for much longer, possibly separating out.
And if it's something like that, it'd just be a funny way to state their situation.