Eq. (14) defines the sum negativity as $\sum_u |W_u| - 1$, but there should be an overall factor of $1/2$ (see arXiv:1307.7171, definition 10). For both the Strange states and the Norrell states, the sum negativity should be $1/3$: The Strange states (a.k.a., Hesse SIC vectors) have one negative entry in their Wigner representation, while the Norrell states each have two negative entries of value $-1/6$. This makes the greater robustness of the Strange states under incoherent noise easy to see, because mixing in the garbage state hits the Norrell states twice as hard.
Eight hundred forty-four!