Look at your flow field. Residuals are a function of the numerical error in the model, but aren't the only convergence check: do the tutorials and you'll be shown monitor points too. Chances are the flow has a number of stable solutions, and it took a while for the bulk flow to establish and then settle down.
To add, I'd be wary of those results and run on for another few hundred iterations. Turn off convergence checking on the Residuals panel.