Why does increasing top pressure of a blast furnace from 1.5 to 3.0 atmosphere allow for increased productivity?

Any furnace is built for a certain working pressure. In general the differential pressure inside a BF (dP) is maintained at 1.4-1.5 . This differential pressure is nothing but the difference between the operating pressure of BF and the high top pressure.

By throttling which is  nothing but increasing the pressure, the productivity of a BF can be increased. The hot blast which enters from the tuyeres in the furnace goes out of the furnace unutilized in the form of top gas as we know it. Thus by maintaining a higher top pressure,the furnace can be operated at a higher pressure and it will result in the utilization of this waste gas inside the furnace itself and as a result there will be more contact time between the charge (burden) and the gas thereby increasing the productivity. ( this can also be understood by relating it with the post combustion as adopted in steelmaking furnaces)