It basically comes from ICAO.
It is assumed that a nation's medicals, licences, Certificates of Airworthiness, and maintenance rules all work together like a complete jigsaw.
Mix and match and it *might* be okay, or there may be some odd mismatch - say a country accepts red-green colourblindness, and mandates a dolls-eye indicator light for the undercarriage. Another country does not accept red-green colourblindness, but mandates red-green gear indicator lights. Both fine as combinations, but one cross-combination and you might get a gear up landing.
That's the basic reasoning behind it.
At PPL level it's all regarded as fairly acceptable to mix and match of-course, just different countries have a different approach to that: basically PPLs are deemed unable to kill all that many people at once. At commercial level, it's seldom acceptable.
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