I’m hoping this is relevant and I will get some more videos up when I can.
Here is a slow motion video of how structural flexing due to high accelerations can cause slight harmonic motion in the nozzle that will show up as a ripple in the layer. Because the harmonic is generally the same the rippling appears to follow the curvature of the perimeter. The perimeter itself is not the cause of the artifact but the structural flex that is induced from the forces created from the nozzle and bed changing directions.
http://youtu.be/a_pDbi0-bEQ
Notice how the harmonics begin at maximum acceleration–directional change–and stabilizes at minimum acceleration–the middle of the run.
Although not applicable to everyone I hope this gives a little more insight into how some types of mechanical artifacts can be created.