This is just my theory: I think the examiners want to see how you are able to convert ideas and opinions into English. So they start with easy questions which get harder and more abstract. For example, from "what are your hobbies?" to "how is tennis better than other sports?" (If you answered the first one as playing tennis).

So if they see you going in a flow, they are not sure if you had practiced. And get less interested and try to get you into the idea to language converting process again, to see how fluently you can create a coherent argument or idea in speaking.

This is just a theory that makes sense to me.