Zou-He pressure and velocity boundary condtions

Recently I’m trying to apply Zou-He boundary conditions for Poiseuille flow problem.
The code I started with is http://wiki.palabos.org/numerics:codes
firstly, I applied velocity boundary condition for the inlet, and pressure boundary condition for the outlet like the above model,
and from the simulation results I got the density of the inlet.
then I changed the inlet boundary condition from velocity boundary condition to pressure boundary condition, and applied the same density,
but I found the simulation results are different, anyone knows where is the mistake?
Thanks !!