The reason for that is the DPF is working harder, creating a lot more heat. There is a trade-off between NOx and soot. Less NOx, more soot. So that meant the DPF has to work harder. VAG says they used newer technology to compensate for the extra fuel used by the DPF and this is confirmed by official German KBA tests that insists the engine delivers the same power and torque without any increase in fuel consumption and CO2.