I've noticed bit of the same thing, but this is mostly in large cities like Oslo and Trondheim.
I believe the reason for this is the amount of people, and generally the Norwegian way of being as others above me has explained.
As a Norwegian from the north, I've felt the same "cut off" behaviour, but I of course read this as not being unfriendly, but just not looking for any new acquaintances at the moment.

It seems to me that foreigners, are kind of always on the look out for new people to get to know, but that's where we're a tad different.
We're more happy and satisfied with what we already have.
I can understand why people form other countries have problems reading this, and feels that Norwegians are unfriendly.

And also, I feel we're more into few GREAT friends rather a lot of friends who you don't know if you can trust or not.

Quality frinds over quantity.

And this unfriendly thing I believe to be way off.

I'll give you and example

There has been these social experiments mostly in USA, you can watch them on youtube, where for example some guy falls over and looks kind of sick or just dead, at least in need of help. None helps, some just walks over and looks, and walks away.

There was a socialy experiment done in Norway to see how Norwegians would generally react to a stranger in need.

A boy alone, maybe 10-12 years old, during the winter, without a jacket sitting on a bench just minding his own business.
If I remember correctly, not a single Norwegian walked past without doing anything, some gave him their jackets until he could get home or something like that, one guy actually went to a store and bought him a completely new jacket.

However I might agree on the fact that we're a bit harder to get to know, but I think that's more of a privacy thing.
But I promise, most Norwegians are actually very lovable, easy going, helpful, considerate and good friends.

Good luck on getting to know Norwegians!
Merry Christmas.