Or, if the manipulator is pointed "into" the source, that is, into the furnace, then why does it show these filters?

After I feed it belts, say with sand, sand is added to the filters, but all the other filters remain. I'm roughly assuming that the manipulator also receives the filters the furnace can produce in general (meaning not just what's currently discovered, but everything the furnace might produce in the future). But why? This isn't the source of the items it takes from; in theory, these filters shouldn't exist at all, since the manipulator doesn't take from the furnace, it only places them in the furnace.