Topic: Info different process time

Dear all, I'm 26, and son of U.S citizen. I'm born in Ethiopia but i'm a Somali citizen. When I was 2 years old my family move to Italy and I was there for 25 years. On March 8, 2012 my sister and I, who she is born in Italy and she is citizen, we came in America. On July 16, 2012 we both applied for I-130, that have been approved. For some reason, my sister could apply for I-485, instead of my that they said I have to wait for bulletin visa. Why there is this difference? we are both children of U.S. citizen, that did the application for us. I have the Italy green card because I'm not born there so I'm not citizen as opposed to my sister. She is 23 years old. I know is confusing but what i'm asking is: why I can't apply for I-485? and how long I have to wait? if my visa will expiry before the time I have to go back to Italy??? Thanks for the help