Outsider resistances

[jasin]

They were given these immunities in 1st Edition (pretty much arbitrarily), and they just kept them and updated the mechanics. The same way that demons were vulnerable to iron weapons and devils to silver weapons.

Well, there's at least some backing in folklore for the cold iron thing: faeries often fear or are vulnerable to iron, and they're chaotic and capricious, and demons are too.

Not sure about devils and silver. It never stuck out as arbitrary, but now that you mention it, I don't think I know of devilish creatures in folklore or mythology being hurt by silver...