I can't figure out how you're getting the price to show up in Krone or in Euros from a US-based departure city like Los Angeles. On the Norwegian Air website, those US-based cities that you can select from as a departure city (in your example, LAX) only show up as choices on the US version of the website. The only way I could see a fare calculated in Euros is by selecting a European departure city (say Algarve-Faro, Portugal, etc).
Or are you thinking "why is the return flight/leg less expensive than the outbound flight"? That seems like the only way you could pose this question - did you look at the return flight cost (LONDON-LAX) in Euros or Krone as a one-way and you're trying to get the same or better price on the outbound (LAX-LONDON) leg? There are a variety of reasons why two one-way flights don't cost the same even though they're between the same city-pair (landing fees, airport specific taxes, etc).
I hope this makes some sense and doesn't add to the confusion. I honestly don't think any semi-sophisticated business would let someone game exchange fates not in their favor since that's what they constantly try to do to us. If you look at some hotel rooms, for instance, there is a local price and then a foreigner price. There's no way to fake being a local because you have to provide a foreign address and the website software code "knows" this.
