Haven't received the EAD card yet but already have an interview schedule (marriage-based)?

My friend was confused on how does US immigration process the applications for EAD and Adjustment of Status (Marriage-based). She just filed all the applications last week of December '08 and done w/ biometrics last February 1st week '09. Now she received a letter from Immigration for their interview on April. Why she haven't received her EAD card yet? Any ideas or informations will help.