are phrases that belong to the default custom field that is created when XF is installed.
If there are no custom fields, then there's a fairly good chance that the translated version of the phrases exists, but the master version of the phrases doesn't. We would only delete the master version of phrases when we delete the custom fields.
There was also an old bug which incorrectly saw the phrase associated with the add-on, this was rectified, but it wouldn't apply to translated versions of the phrases.
The short version is: If you no longer have a Custom Gallery Field with the ID "caption" then you can revert those phrases. They should then disappear. Either way, the current behaviour of them not being exported is likely correct due to the reasons I mentioned above.