There is also the fact that, by the moment you know your way around, ruts form, and you actually end up seeing only 5% of an area: the part that you have to go through.
When I lived in Milan I made en effort to take as many different (but reasonable) routes to go to school (or to work later).
Despite my best efforts, though, there are parts of the city center that I have never visited. Maybe one should draw a huge circle on a map, and then walk the streets that best approximate that circle, forcing himself to visit unfamiliar places.
In many parts of the world that could be suicidal, BTW.

Anyway, one of the things that I like best about getting to know someone is to be shown their version of the city: what is their idea of this place we share.

This particular phenomenon could also be linked to the general ease with which we slip into routine: some times try lying on the floor or climbing a chair, and survey your living room (or bedroom). It will be a different place, trust me.
Routine vision is probably the deadliest infirmity for photographers.