Fluid flux across "0" flux boundary is not zero

Hi! I'm a new user of FEFLOW. In my model, the northern boundary is impervious. So I didn't prescribe any boundary condition at this border, which means the northern border was set as a "0" flux boundary.
But when I check the flux across the northern boundary using the tool of "Fluid flux analyzer" after running the simulator, the value of fluid flux across the "0" flux boundary is not zero. So I'm very confused why the flux is not zero.
Looking forward to your reply! Thanks!

The fluid-flux analyzer (in FEFLOW 5.x and 6.0 Classic) does an integration of the flow based on the resulting velocity field. At no-flow boundaries, the velocities that are calculated are typically not exactly parallel to the boundary (due to spatial discretization). In this case, the fluid flux analyzer will show inflow or outlflow at such boundaries. The finer the discretization at the boundary is, the lower the flow values should be. In general, it is better to use the Budget Analyzer for checking on boundary flows. The fluid-flux analyzer is intended for internal flows where the methodology of the Budget Analyzer does not work (yet).

Thanks for your reply! I have got the idea!