Although the answer is not in the AoS
core rules, it is in the section "Warscrolls - Hints & Tips" of various publications, e.g. every Battletome.
Random Values: Sometimes, the
Move or weapon characteristics on
a warscroll will have random values.
Generate any random values for a
weapon (except Damage) each time it is
chosen as the weapon for an attack. Roll
once and apply the result to all such
weapons being used in the attack. The
result applies for the rest of that phase.