A girl in my sorority got assigned two "bigs" last night? Why does she have to and everyone else only has one?

We picked our top four girls we wanted to be our "bigs" and emailed them to our new member educator. Then this week we received presents and stuff from our big and when they revealed themselves, she got two bigs that she put down on her top four and some of us didn't even get a girl we wrote down on the email. Why is it that she got two?