Currently, we have some tags which can be used to indicate what sorts of data answers will work with: number, string, array-manipulation, date, and so on. One of those sticks out from the rest: array-manipulation, which has
-manipulation unnecessarily appended.
Interestingly, array does exist...but as a synonym, while date has date-manipulation as a synonym (and number and string have no synonyms at all). This is obviously inconsistent. The added
-manipulation doesn't add anything useful, as array would still clearly indicate a challenge involves the manipulation of arrays, just as string indicates the manipulation of strings.
I doubt there's any intentional reason for this; array-manipulation was created a while after number and string, and it's likely nobody really thought about it (though it evidently confused some people, if array was created and made a synonym).