Different setups. Direct injection is as it states. You have the injector pump, injector lines then injectors. Common rail, have a fuel rail, in between the injector pump and injectors. Im not up to speed with these but I think the fuel rail is what controls the fuel delivery pressures to the injectors, where on a Di, the injector pump does all this. The injector pump on the CRDs pump the fuel through to the rail and injectors at a much higher pressure than a direct injection motor, hence giving a slightly better power delivery and better fuel economy.