It looks like Mami's neck was already completely cut through (notice how she keeps her hand gripped around her neck for most of the comic?), and she was managing to survive through sheer force of will and hotbloodedness. However, in performing Tiro Finale, she has to use both hands which removes the hand which was keeping her head connected to her neck, and then, I don't know, the recoil probably finished the job and completely separated them
