I would guess it depends on position of missile detonation in relation to target position, is it infront, behind, or to the side/up/down.
If it is behind, I would go with farokh's 5m because of the shockwave, but depending on target speed this could do little damage.
If it is in front, then 12m, as mdore suggested shrapnel spreading.
If it is to the side than 5m again, as 12m shot would have shrapnel fly behind a target (taking 60 degree shrapnel cone)
Of course the more I typed the more I felt it has nothing to do with the warhead, maybe it is a trick question. Then maybe Im just getting paranoid.
The question refers to approaching targets.
You are right , it has nothing to do with the warhead but with the proximity fuse .
Proximity fuse is energized with k3 and always in a standard and fixed way (receive certain number of frequency returns from the target).
In our case the position of the target in relation to the axis of the missile is the KEY.
The missile ,if the target is inside the 60 deg of proximity fuse,should detonate always in a fixed distance .
In first missile we have 5 m detonation (miss) distance which means the target was out of the expected window of proximity fuse and some how the missile get much closer of the expected optimum position and then received the required returns from the target and detonates.During this the missile maneuver and forced for better position and so the probability (parameters) for a kill ,is lower.
The second missile performed much better than the first one. It acquired the target with prox.fuse in optimum range and so it is expected a better kill probability.
The closer is not always the better when there is a directional warhead .
Thanks for your participation .