Generally, the Best Screenplay categories are filled with Best Picture hopefuls and, when there is room, one or two “honorable mention” choices. These are films that don’t necessarily get the love they might deserve in other categories, but the WGA and the Academy want to honor their merits in some way.
In the Original Screenplay category, this is something like The Big Sick or 20th Century Women. Both are great films that could have competed in other categories, but for one reason or another didn’t quite make the cut. For whatever reason, the screenwriting branch seems to be more forgiving of these clever little films.