Some series had episodes produced that were never aired, but you can still obtain and watch them. https://www.tvmaze.com/shows/27591/the-treet/episodes for examples has several. The episode list shows the unaired episodes correctly, but the watch list doesn't show them, and once you've watched all aired episodes, the whole show isn't shown on the watchlist any more.
That's kind of inherent to the watchlist, where episodes are only listed once they have aired. If we'd allow episodes without airdate to be listed there, you would also suddenly see episodes that aren't aired/available yet at all.