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Assuming you're just monitoring an area/mass average on the outlet you may be missing some flow features in the domain. 

Remember, a residual is a function of the solver error. So, your initial condition could be really good; or (more likely) there's an unresolved flow feature or transient in the solution. For the latter point monitors will pick up more information, as will taking contour/vector plots every 5-10 iterations for 50-100 iterations. 

What you may find is the fluctuation/error/something is negligable. I always cover "poor" convergence in training: check monitors, fluxes and residuals. Then think about what each is telling you.