I guess the information that's missing is what your actual salary is. Perhaps it's so low that they consider that you can't possibly live on that in Norway?
Ok, let me put it another way: the US does the same. Tax you on foreign income.
"U.S. citizens and resident aliens are taxed on their worldwide income. You must report your wages and other earned income, both domestic and foreign-sourced, on the correct lines of your Form 1040."