Board Thread:Speculation House/@comment-10380701-20141011082048/@comment-24891101-20141015221729

Presumably blunt force trauma is easier in general for Aura to disperse, by virtue of being spread over a wider area. Sharper things are generally more dangerous, and so they might locally overwhelm her Aura, leading to wounds, so she avoids those, but disperses and absorbs part of blunt force impacts.