While going through CC training I remember a weapons instructor (I think it was) mention it was to minimize roll inertia when you get within visual range, where maneuverability is more important than when you're BVR. Typically you would have gotten rid of the 120s by then and having the mass of the 9s one station closer to the centerline reduces roll resitance a little.
Was more of a side comment, so we didn't think more of it at the time but, I doubt that's the main reason. Esspecially not enough to offset the increased fatigue caused by the CATM 120s on the tips, and even more so when pretty much all we fly is peace time training or A2G on deployments. Occasionally we heard they wanted to fly with a certain loadout so the pilot could get used to the handling characteristics, but with heavier loadouts. If having the 9s on either 2 of the outer most stations really mattered I'd assume we would have also flown VVR fights with only CATM 9s and empty tip rails.
And currently studying aircraft structural engineering I wince mildly when thinking back on the twisting and bending those tip 120s did to the wing.. the 9s as well though for that matter..