His name is actually Stefán Karl. Stefánsson is not his surname, in fact, it's not really a name at all, it's just a patronymic title (as TightenUpTh sort of explained already). Bleh.
They film the show in English first, then Stefán Karl (the guy who plays Robbie Rotten) and Magnús Scheving (the guy who plays Sportacus) record all their lines again and they re-dub the whole show into Icelandic for the home-market.
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