This from AI, remember AI can be wrong but I think it’s on the mark:
• 93 is an inland grade. It is sold on the Highveld because the air is thinner. Johannesburg sits around 1 700 m, so atmospheric pressure is roughly 15–17% lower than at the coast. That lowers effective compression in a normal engine, so most cars knock less and do not need 95.
• The coast only sells 95. Denser air raises cylinder pressure, so 93 is not offered in Durban or Cape Town.
• 95 is for engines that ask for it. Turbos, superchargers and high-compression motors (many TSI, EcoBoost and similar) still need 95 inland, because the turbo makes its own pressure. A knock sensor may save the engine by retarding timing, but you lose response and can use more fuel, not less.