Consider two people, A and B, living in two different cities, where A is north of B. They're talking, and B invites A over to his place.

"If you get a chance, you should come up here sometime!"

But B is south of A! Should he not have asked A to "come down here" instead? In my reference basis, north consititutes "up" and south is "down" ... like you're holding a map (right-side up). And yet, I so often here people use "come up here", where "up" refers to their own location or city. Perhaps it's a subconscious instinct for people to assume that wherever they are is up or higher than wherever else.