I wouldn't read too much into it. Everyone that's getting a call IS in the firm's good books. As long as you got a call, you're in a good place.
It will depend on the firm how much instructions are given to callers. For example, some callers might be told to book certain candidates on Monday or even Monday morning. But other firms might tell their callers to call in order of the list with no other requirements. This means that you might get asked "can you come in at 8/10/12/2?" or "what time do you want to come in?" In that sense, it can be arbitrary because they just want to schedule you in.
Disclaimer: I'm not a caller, but this is my understanding of the situation. Correct me if I'm wrong.