That's the main reason you get lower vent temperatures at low fan speed. Think of it like this, if you double the air flow through the evaporator you may increase the heat transfer by 50% and remove more total heat from the cabin. However if you increase the heat transfer by 50% and the air flow by 100% the final vent temperature will still be higher.

Having outside air flow will also affect the temperature, especially on very hot/humid days. There is a compromise between having maximum cooling and not having to breath 100% recycled air. Don't forget that back in the 80s some people still smoked in these cars, having some outside air helped prevent them from killing their passengers as quickly.