MRT Zou/He pressure boundary condtion

all dear:
I want to validate the Poiseillue flow with MRT and Zou/He pressure boundary condition, but meet over the a preblom that the difference between analytic results and MRT are bigger than the ones between analytic results and SRT at the same zou/he prssure boundary condition. i don’t know that cuase. i use the same relaxation time, 1/tau,in MRT and gain the same results with SRT, 1/tau.
I need all helps. How to deal with it? what is cause?
In an advance, thank you a lot.