They all work fine because Ghost, Dragon, and Bug are broad terms.
They're a variety of different ghosts. If someone where to see a spooky apparition I guarantee many would identify it as a ghost before anything else. No one is going to mistakenly identify a palm tree as grass.
Dragon is also a broad term and there are so many different types of dragons that the type name works fine.
While the actually definition for a bug is far narrower than what most people assume it's become so common place to refer to any creepy crawly arthropod as a bug that it works fine. Besides, something like Insect type in that case would still incorrectly label Ariados and Galvantula and Arthropod type is too bulky to work properly.