It's coming from the language strings.

It would need its own string if its a column heading instead of reusing a generic string

It's not a heading, it's the content in the row.

ok - deosn't need a period then BUT these strings already existed and are in a generic file so it is reasonable to assume they could have been used to produce something like
File named john was created 23 hours ago.
So if you want to use it here without the period then it will need to be new strings

Same strings are also used on the privacy component's requests list. Just something to keep in mind for whomever works on a pull request.

And any 3rd party extensions that use JHtmlDate::relative().

@Quy can we close this one ?
it seems lack of interest