Full Defense and Willpower?, Why Willpower and not Reaction? Options

[j2klbs]

I love the idea of Interrupt Actions! But I am confused about "Full Defense".

Per preview #2:

"Reaction - Reaction is about reflexes, awareness, and your character's ability to respond to events happening around them."

"Willpower - Willpower is your character's desire to push through adversity, to resist weariness of spellcasting, and to stay upright after being nailed in the head with a sap."

Per preview #4:

"Full Defense - A character may choose to focus on deliberately avoiding incoming attacks.... This action adds the character's Willpower to their Defense tests for the entire Combat Turn."

What??? 8-/ In my mind, the Reaction attribute makes WAY more sense to be used with Full Defense than the Willpower attribute. Can someone please explain to me with how the attributes are defined why Willpower should be used instead of Reaction. It doesn't make any sense to me.

Thanks!
~j2klbs