Difference between postdoc and researcher in Sweden?
The difference is that a postdoc's PhD degree must have been obtained no more than three years prior to the application date.  For a researcher, there is no such limitation.

It means they want a postdoc, but if it's been more than three years, they can't call her/him a postdoc and will call her/him a researcher.  They'll probably pay more for the researcher but they probably would also pay someone with 8 years of postdoc experience more than someone with 2, even if they were allowed to call both a postdoc (like they would in, for example, the UK).

It's common in Sweden.  See this question.