Nope, wrong conclusion. When you email (or use Send to Kindle app) Word document to Amazon's servers it gets converted and the book / the document is delivered to your Kindle in one of the "native" Kindle formats (mobi, azw3), but when you've users PC and USB cable to directly copy .doc /.docx file naturally there is no format conversion.
Kindle itself doesn't support opening Word files, Amazon's conversion & delivery server does accept that format.
You can easily verify my claim if you connect your Kindle to your PC and examine contents of "documents" folder and search for file with the (file) name / title of that book /document you've emailed. Check the file extension and it will probably be ".prc" (or .mobi, .azw)not ".doc", ".docx", ".rtf".
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