Stockholm Västerås airport is also called Hässlö. It is an old air fleet that Västerås municipality had to take over. It is an "extra" airport to the Stockholm region, which is why it has Stockholm in its name.
Ah that makes sense. I was quite surprised how far it was from Stockholm, I know Ryanair tend to use more localized airports but it took us 2 hours to get to the city (admittedly there was a lot of traffic!) Thanks for the information.