I have found classifying roads into primary/second/tertiary in the area extremely difficult.  Due to the hills roads take a lot of non direct routes.

The classification mainly changes what zoom level they render at and gives an idea of how useful they are to know.

As a rule of thumb, I was going off speed limits which is why I initially downgraded it.  But you're right it probably should be a secondary cause it looks weird to have a tertiary in-between two secondaries and despite it's speed it's heavily used.