> How is the pattern chosen so that all the lines meet up?

Trick question! It's random.

> What happens if you delete "1 setlinecap"? Is there a minimal fix for that?

You get little breaks in the circles.

"1 setlinecap" is not really needed if I change the file
to "0 -1 moveto -1 -1 1   0  92 arc" and "0  1 moveto  1  1 1 178 272 arc"