How Does Shunt Placement In Heart Help In Increasing Oxygen Levels In Blood?

In Tertralogy of Fallot, after Systemic to Pulmonary A. Shunt is done the mixed blood goes through Pulmonary A. then to lungs to get oxygenated.
This blood gets oxygenated then enters heart through Pulomonary V.
Now isn't it the same mechanism happening? So hw does this method help to increase the oxygen level in blood?