Some helpful background for the data presented by Petraeus:
Attacks against "same-sect civilians," U.S. forces, the Iraqi government or Iraqi security forces "are excluded and not defined as sectarian attacks." So even though Sunni insurgent groups loathe the Shiite-controlled government, insurgent attacks on it aren't considered sectarian violence.
If no attacks on the Iraqi government forces are defined as sectarian, we have a problem. Full FOIAed report here.