@EniesLobby7 Read the thing directly to the left of "Completed." That section of the page is specifically to tell you if the series is done publishing in its country of origin, while the individual translations (since there could be more than one language on this site) will be tagged with "END" on the last chapter if that's the ending.
The way you should make use of this information is along the lines of "So, this series is done but nobody's finished translating it, and the last update was years ago? Well, guess this is dead as doornail." Or "The series is completed, and it looks like the translation is being updated frequently. Guess they'll catch up sooner than later." Maybe a "So this one is still ongoing, but the translation seems to have stopped a long time ago. Seems like a waste of time to get invested in it now."