perhaps 80% top notch applicants are accepted as first choice, the 20% less qualified applicants are given a 2nd chance before they offically announce the result on its web.
The other reason maybe the school is interesting in meeting the parents of their preferred applicants.
relax. you are only a week away to find out. some moms got call couple days ago, and I got mine this morning. perhaps you will get yours later this afternoon or earlier next week.