Amazon doesn't always use the file extensions consistently between different devices. Books downloaded using the Kindle for PC program will have an azw extension regardless of whether they are actually in mobi or azw3 (kf8) format. Calibre will change the extension on import.
If you download books to your Voyage and try import them into calibre, some books will not import because the latest Voyage firmware supports a new format, kfx, which calibre cannot handle.
Also, you may want to read
the thread I started based on my experiences with books being updated after purchase. Amazon will only push updates if a book has major errors. If minor changes are made by the publisher you will not receive them unless you ask customer service, and they don't provide any way for you to know that this type of update is available unless you ask about a particular book.
Personally, for books I purchase on Amazon I let Amazon handle the delivery and don't use calibre except to make a backup copy.