the data isn't periodic. its just a stupid display choice that actually makes it harder to see the slope of the graph
The circular areas are presumably meant to suggest the number of artists at each level. The innermost circle contains only 1/7th of the total number of names, etcetera. (Though this would have been more appropriate if each ring had increased by the square, instead of linearly.)
probably because according to pi * d, they can squeeze more data in the chart and still fit within the width of that layout column.