Why are these filters producing different results?
I actually figured it out - as it turned out, I missed a small but important detail. The original (local) filter included a relationship to a content type within the view, which was providing the correct results. When I went back to edit it, I'd noticed that when I created the contextual filter, I hadn't provided the relationship to the filter. Upon adding the relationship, everything worked out the way I'd intended - getting all the expected results. I still don't know why I was getting that unexpected behavior, though.