typical sorority girl

I see the phrase "not your typical sorority girl" used on here a lot. I have rad through many posts on the discussion page and on the sorority page. When I see people write "not your typical sorority girl" I get the sense that it is being used with a negative connotation. What exactly do people mean by "typical sorority girl." From what I have seen at UT is there are so many different girls that are in sororities so what is "typical." And why the negative connotation?