Pressure contour near outlet boundary is variable

Hi, everyone,

I employed the COMSOL Laminar Flow model to simulate parallel plate flow. In the Laminar Flow model, I exerted inlet pressure boundary (a constant value) and outlet pressure boundary (another constant value). It runes well in this simulation and didn't get any warning or error. But I came to find that the pressure contour near outlet boundary is variable, not constant. It's attached below. Who knows the cause and cure. Thanks a lot.