Blood Most Potent

Question: Why do Njord, Freyr, Freya, possibly Frigg and Idun and any of their descendants have Jotunblut? I understand mechanically, but mythically, it seems like they do not have the blood of giants - the point behind the PSP. Is everybody just assumed to be a descendant of giants at some point if they hail from the land of the ice and snow?

Mythically, we actually have no idea where the Vanir come from. Their genesis isn't discussed, and their bloodlines aren't really clear to us other than the understanding that they aren't Aesir.

However, according to the Norse creation myth, Ymir was the first giant and all the gods were descended from him and his son and grandsons, so technically we would have to assume that the Vanir come from somewhere in there, just without the explicit genealogy we have for their Aesir cousins. Vanir tend to be much more concerned with nature, fertility and the earth than the more sky- and civilization-focused Aesir, so they might be a branch descended from the giants in a different direction, perhaps via Jord or another similar figure. If that's the case, they have as much right to Jotunblut as the Aesir do. If you want to get creative, you could also decide that one of Odin's brothers, Vili and Ve, might have been the founding father of the Vanir just as Odin is the founding father of the Aesir, considering that both of them also vanish from Norse myth shortly thereafter and their doings are never discussed.

Of course, you could decide that the Vanir come from some theoretical other provenance that doesn't trace back to Ymir, and even if you do believe they're from somewhere in Ymir's line, that doesn't cover everybody (Idun, for example, is apparently originally at least part alfar, since she's included in the descendents of Ivaldi, and therefore might be a deified lesser immortal rather than of the same bloodline as the other gods). In those cases, you might want to roll with the idea that the Vanir that live among the Aesir (i.e., the only ones that are playable and have Jotunblut anyway) have been officially adopted into the pantheon and their PSPs accordingly changed from whatever the Vanir originally had to match their new pantheon's giant-sized blood. It's theoretically possible for a Scion or god to switch pantheons, provided that they do so through a very important and difficult event involving the Wyrd and the complete abandonment of their original pantheon, and the permanent trading of Njord, Freya and Freyr to the Aesir might have been just such a case (not to mention Sif, Frigg, Sigyn or any of the other possibly-Vanir ladies). It might just be that everyone who permanently lives with the Aesir has been fully converted to join them, while the largely nameless or vague Vanir who remained in Vanaheim retain whatever their original powers were.

Of course, the simplest reason is "because the Vanir and Aesir are not mythically developed enough to be different people and they're both just classes within the Norse pantheon anyway, so it would be mechanically difficult and silly to give them different PSPs". But if you or your players are looking for an in-world reason, there are plenty of options within the framework of Norse myth. This is one area where the general vagueness surrounding the Vanir works for you instead of against you; there's nothing that tells you explicitly where they came from, so you're free as a Storyteller to come up with a theory you like and roll with it.