I have wondered about this for awhile...we live in des moines, but, you can address mail to us in, seattle, burien or des moines and we will receive it. our area used to be unincorperated until a few years ago.  we were annexed by the city of des moines.  my guess is that while we were unincorperated burien was our official (?) post office. even though we were annexed by des moines the postal office still tries to say we are part of burien...( confusing? you bet! )

I think the main thing the PO cares about is the zip code. I bet you could put Seattle for anything in the greater Seattle area and it would still get there as long as the zip code and address were correct.

I have sort of a similar situation. I am right on the town line.  My house is technically in Hollywood MD.  That's what my survey, taxes and appraisal say.  And what the house listing said when it was for sale. But the Hollywood PO is small so our street is surviced by the Leonardtown PO.  Hollywood uses the Leonardtown Schools.  But if you addressed someting to my street and Hollywood with that zip I would still get it. 

Our place is the same way. If I go to usps.com and click Find a ZIP Code, I can enter our address with either San Diego or Coronado. Both will come back with the city name I entered but they give the same 9-digit ZIP code.

The Zip code is the most important piece of information.

I have all my books mailed to my work address--just yesterday, I received one that was addressed (hand printed) with my name on the first line, my company name on the second line, and the city state zip on the third line.  No where was my street address, but the book still made it to me.  (I live in a big city too--Sacramento.)  I was awfully surprised that the book made it here at all!