I previously read someone's take on this in relation to people feeling secure enough to take risks here (not just in Stockholm but Sweden as a whole I guess). While notch's success may be an outlier, the process of trying to do your own thing isn't here. I'm not sure when in his development Notch left King, but in general there are quite a few people here who just try to go their own way (sometimes completely outside of their previous industry even). It helps when you don't have to live in fear of being completely penniless should you fail, I think.
Purely anecdotally, I am not part of a startup, but do a lot of hobby work in coffee shops around Stockholm. I'm always seeing/hearing people in local cafes discussing some new startup or project they are getting off the ground. I've also seen first hand many people get maybe a year or two of funding and start up their own game studios, for example. Of course not all of them end up being successful, but many people feel secure/comfortable enough to try. I think the volume of these attempts and people trying to do something new also helps drive up the number of "hits" overall, to be added to these kinds of lists.