Dithered images are not properly dithered

When looking at creating dithered images (PIPE2D-581) I noticed that dithered images are looking suspicious. arnaud.lefur and I did some investigative work, presented in the following figures:

1) Dithering_May14.png - shows by how much the center of flux changes between images after dithering. I created results for the sequence of only 4 dithers (4440-4463) and the sequence of 9 dithers (4720-4773), taken at Subaru. Also added is the data from the last LAM dithered acquisition (4 dither positions, 21256-21279). I conclude 3 things:

a) Subaru and LAM results are different 

b) final LAM dataset is also not perfect - at some point in the past the values for dithering were deduced by arnaud.lefur so that dithering was ``perfect''. 

c) there is also potentially the difference between the results when depending on the sequence of moves (the difference between yellow and black data), but this would have to be further investigated in order to make a certain claim.

I am also attaching Figure Dithering_May13_Arnaud, which shows the analysis did by Arnaud which mimicking the black points from the previous plot. The results are consistent.

Finally, I am also attaching Figure Dithering_pixels_added_simulation.png, which was the original Figure showing this effect that was discussed on Slack.