Axial Velocity Calculation

Roux

I have performed a CFX simulation where the Inlet BC is Total Pressure and Exit is mass flow rate. When I measure the Axial Velocity component in a cylindrical region before the exit (at 2D lenght before the exit), the values varies by as much as 20to 25% to that of the Actual value (Q/A). The change in mass flow at the same location is very small and I have used massflowavabs to calculate the AXial Velocity. Why does this happen?

Thanks you