The version of postgres you think you're specifying isn't exactly the community postgres, it's an AWS maintained fork and this is an example of where they were seeing an error message from a future version of postgres that had been back-ported in the fork (or maybe AWS contributed to PG and it landed in a later version).
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I guess I knew this - it's obviously not the exact same version, when you stop to think about it (pretty sure the doco tells you this explicitly too).
But I've never personally observed a difference, so I never really gave it a thought.