Asked to attend in person in interview after getting 221g Status

I have got white slip during my interview and my status has been refused (221g Status). After 3 months I have got an email asking me to attend the interview in person they have given me the time and date of interview in Chennai. I have not got any message for the dropbox . Please help me to understand why they have not asked me to put in to drop box and what for they are asking me to come in person. Is there would be any reason?
Thanks in advance.