It might depend on how sensitive their radar is or whatever. It could just be a slight difference that makes it a hole or a misty cloud. I at first, couldn't figure out what the bizarreness was about.
There is another effect that I happened to witness myself. And strangely enough, it was after watching a program on it. It's where clouds with enough moisture in them, and the temperatures are cold, freeze in the middle of a formation. Then you see what looks like the middle dropping out of the layer of clouds and it "rains" frozen clouds. It leaves a hole in the middle. I've only seen it once.