Spinal Flexibility and Deadlifting

A question regarding flexibility of the spine and its relationship to deadlifting. Notice the slight rounding of my lower back as I begin the pull. This is a new style of deadlifting I am using which has my back flatter than my normal pull. It seems I am unable to keep my back in a locked arch position, although the rounding is actually less than what I am used to and much less than I am capable of.