Which way was the plane rolling?
Because I might make a wild guess that it's actually rolling LEFT wing down i.e. in the direction that agrees with the diff-stab deflection. Perhaps, at Hogh AoA, the aileron effectiveness actually reverses (it is the case that at high AoA aileron effectiveness CAN reverse; whether it does on the Hornet I don't know). In which case ALL the surfaces are commanding a consistent left roll; they just aren't deflected normally.
If the roll was to the right, it's possible that the stab position is in some way related to relieving v-tail loads or improving rudder effect; but that seems a thinner explanation.