Err, I deduced from the fact that those files got mentioned at all, that calibre was involved???
I consider it perfectly natural for someone to ask a question about
calibre and Kindles, in the Kindle forum. It could very well be that this turns out to be more about calibre than Kindles, or possibly
about Kindles, tangentially related to calibre.
Since it is calibre that created those extra files on export (save-to-disk will save them in addition to the actual ebooks), I consider it
entirely natural to assume that anyone discussing the copying of ebooks
in the context of this thread, to be referring to calibre.
...since, after all,
calibre is about Kindles (and many other things)...
Sheesh!