Well, it's not necessarily. However, combining elements of the two disparate cultures into a single autonomous society is far more interesting than simply carbon copying one or the other with slight tweaks and twists thrown in to make either seem, "original,". Headhunting tribesmen protecting sacred groves and standing stones garbed in the bones and skulls of their enemies wearing painted, wooden masks and howling wildly to the beat of drums as they whirl about a burning wicker man in the light of the setting sun, rattling rain sticks carved of petrified wood filled with the knuckles of their ancestors whose sound is nearly drowned out by the screams of their dying, burning foes seems more interesting to me than, "this is my society. It's basically the Iceni celts, but I tweaked the names a little so now we have, 'drudai,' rather than druids,". 

EDIT: 

This of course is what their society was like before the introduction of Aed and the Southern backed monarchy, as well as the current state of the most zealous keepers of the old ways. My monarch certainly isn't running around in face paint burning people in wicker cages and collecting heads. Then again, everyone but the most extreme pro-Aed or pro-tradition is somewhere in between the two cultures.