The reason is Koenigsegg made less than 20 cars. The production car in context of the list is a car of which not less than 20 were originally built by its manufacturer to the same or faster specification than the record setting example. This list uses the same definition as the List of automotive superlatives for the sake of consistency and because the term production car is otherwise undefined. It is also limited to cars that were constructed post World War 2 cars in the same manner as the List of automotive superlatives.
On the articles talk page you will find a brief mention of the Koenigsegg CCR, but more importantly a debate about the 20 car limit. I hope that is helpful.NealeFamily (talk) 19:52, 15 August 2013 (UTC)