Why do we check the common femoral vein for DVT at the level of the greater saphenous?

59 year old female who presented with chest pain in addition to right leg pain, her right lower extremity DVT study revealed the following:

View of a clear right common femoral vein, but large thrombus in the greater saphenous vein.  Patient was found to have a pulmonary embolism on her chest CT and was admitted to medicine for anticoagulation.

View compressing right common femoral vein showing no clot, but incompressible greater saphenous vein with large thrombus.