Board Thread:Help/@comment-28398720-20160525161708/@comment-26247132-20160525173146

The Sky People/Arkers never refered to themselves as anything other than "Citizens of the Ark" and other variations until the end of Season Two (Resurrection) when Harris tells a joke calling themselves "Arkers." Prior to that, the only reference was from the Grounders calling them "Sky People."

So, the wikia needed a name for them besides "Citizens of the Ark" and used the one that had been spoken on screen. Same for the 100: in order to differentiate between the 100 delinquents and The 100 tv show, the wikia labeled them "Delinquents." That's the shortened history behind the naming conventions if that helps clear things up.