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I believe this is to the fact that he is never "independent" when revolting, and since Sweden is Catholic (the top liege), he doesn't receive the Pagan defense bonus. If you grant him independence, so that he actually is his own realm, the bonus is then applied as it should.
Well. Not really. He might not have a liege at the moment, but he still technically belongs to the realm of Sweden. Besides, if revolting pagans did get the attrition bonus, defending against revolting vassals would be way to difficult as smaller nations. Which is probably the deciding factor as why he doesn't get the bonus.