Hi Ralph,
can't find any documented rationale. The Exocet/Seacat Leanders had more Seacat launchers than the Ikaras and the air threat was greater than the submarine threat.

What is interesting that even though we were obliged to divide our ships between the Carrier and Amphib groups we had significantly more Exocets at sea than the ARA (I calculate 56 vs 20). Thank goodness such a ship to ship missile vs missile encounter did not occur