As someone who is early on in ther career, this is good food for thought. I am still employed at the first company I worked for after graduating from college 2 years ago. I was always told that the way you get a raise is to "job hop" every couple years.
The way I see it, you're worth more to your current company. You already know their stuff, and don't have to get up to speed. And you know the stuff you made for them well. And to them you're not an untested new hire who could flop. So in a rational economy, on average and when you're a good fit where you are, you ought to get the best offer from your current company.
Other companies are willing to pay you more because they need your expertise and you are offering a higher level of skill/experience than you offered the company you are with now.
Because they don't know what you're currently earning, if you don't tell them.
If you know any Norwegians, you can search them up on http://nrk.no/skattelister2008/, for instance.