It is not clear to me why some papers use one or another term. I think they are the same thing: maybe atomic multicast is actually atomic broadcast that is implemented using IP multicast (like ring Paxos).
In distributed systems theory, it is not related to using IP multicast or any other implementation detail. Actually, most of the time it is a matter of personal preference of the author and you can safely assume that they mean the same.
In detail, to be strict, when you say multicast you are assuming that not all processes are necessarily intended to receive all messages. When you say broadcast, you are assuming that all processes are targeted by all messages. The ambiguity arises as follows: As often multicast algorithms are layered on top of membership management, an abstract protocol that multicasts to all members in the context of a view is pretty much indistinguishable from one that broadcasts to all processes in the system model. Hence, you can describe it as either multicast or broadcast. It really depends on the context.