-tagsfromfile creates tags in target that aren't in source?

[camner]

Because video editing software (such as iMovie) obliterates overwrites many of the original tags (such as the shoot date of the video clip), I've been using -tagsFromFile to copy back the tags from the original source to the edited video.

What I have noticed is that after using -tagsFromFile, the target (edited video) file contains many tag groups of the form xmp-something_or_other, but these tags don't exist in the source! 

This confuses me, as I thought that -tagsFromFile would not create any tags in the destination unless they existed SOMEWHERE in the source. 

[FWIW, if I specify -all:all, then this doesn't happen.  No additional tags not in the source are created in the destination]

In case this matters, I have checked and there are no .xmp files in the source directory.

[StarGeek]

Exiftool may not be able to copy the original tags to the original places, especially in the case of video.  For example, if the tag is a Quicktime group tag, odds are that exiftool won't be able to copy it directly, as it can't write many Quicktime tags.  It then tries to copy it a tag it can write, which is often XMP in the case of video.