I know that Jason Rothenberg said that she was floated off-screen after the pilot, but it's impossible: At the start of the story, on the Ark lived 2,659 sky people. After 102 (Including Raven and Bellamy) people were sent to Earth, on the Ark were 2,557 sky people. During the Culling died 320 sky people, it means that there now 2,237 sky people on the Ark, Jaha confirmed that in Contents Under Pressure. Callie could not die before The Culling, because if she died, on the Ark would be 2,236 sky people. Someone should change this.
When they were writing the story they didn't take into account that they were going to end up having to cut out one of their main cast. Slip ups like these happen, wasn't really much of a point in rewriting the script to change some numbers.