In most cases, it’s because the *download* has finished… the file has transferred to your PC. But it’s usually either saved into memory, or more usually saved to a temporary location
It then takes a few seconds to verify the file (to make sure it hasn’t been corrupted) and to save it to disk/rename/move it to the right location.
It’s rare for this to be taken into account in the download/saving percentage, because the actual physical download is complete.
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