norms

help! quick question

Im working on a CFD project for school here and I am expected to do some analysis of my results. When I plot the log of the l2 norm of the change in solution vs timestep, the plot is not constantly decreasing. There are some oscillations. The code is correct as it does converge to the exact solution, and overall the norms decrease, but does anybody know what causes these oscilations. Why would the norm decrease, increase a little bit, then keep decreasing? This is a 2d navier stokes solver on a square uniform mesh for simple channel flow. I am just solving for pressure,x and y velocities and plotting those norms. Thank you very much