Why British convoys still needed heavy cruisers

Why did the Royal Navy assign heavy cruisers to Mediterranean convoys when light cruisers might defeat Italian heavy cruisers?

The heavy cruiser provided insurance against an Italian counterpart when commanders could not risk discovering that the favorable paper matchup was wrong.

Heavy cruisers were not pointless in Mediterranean convoy operations. Their value became obvious if an Italian heavy cruiser appeared. British light cruisers might defeat that opponent on paper, but wartime commanders could not safely base convoy survival on the favorable theoretical case. A British heavy cruiser therefore backed the light force and reduced the risk that one unexpected contact would overturn the escort plan.