They have more to schedule. As I was also told, there are more slots available, and hence more calls to go out.
To answer you, as to why haven't they called already, see, the applications are sent to various faculty members, and they have students to assist them. The notifications are to be sent out by these students. So there are two variable at work here: the professor who takes her/his own time in coming up with a short-list, and the student who may delay sending out the email. I doubt that any student would delay sending out the mail, so it got to be the professor who is taking time in compiling a short-list.
To answer you, as to why haven't they called already, see, the applications are sent to various faculty members, and they have students to assist them. The notifications are to be sent out by these students. So there are two variable at work here: the professor who takes her/his own time in coming up with a short-list, and the student who may delay sending out the email. I doubt that any student would delay sending out the mail, so it got to be the professor who is taking time in compiling a short-list.