Its the fuel rail design...its due to the fact fuel is fed in one end and regulated out the other...an easy trick...try putting the garden hose on the fuel rail, using a small cork with a hole drilled through the centre to act as a restriction (regulator) you will see why No.6 and even No.5 runs lean. The pressure differential over the rail is uneven, thats why a dual fed centre regulated rail will always perform better and even out the pressure over the rail.