I just remembered that when Christianity was trying to spread to the Norselands, they tried to make it as "user-friendly" as possible. IIRC, the Norse were hesitant to give up their old ways, so they used the Norse mythology as a precursor to the Old Testament; after all, once Ragnarok happens and all the old gods die, the only creatures left are one man, one woman, and one tree they had hidden in. I can see how they'd parallel that with Adam, Eve, and the Tree of Knowledge to try and make the Bible more believable to the Norse.
(Although I suppose that, under that context, Ragnarok has already happened so Odin is dead, and he doesn't hold feasts in Valhalla anymore, but maybe Harald was more "Christian-in-name-only" than anything, I don't actually know how devout he was.)