Why every point is in exactly one pair in Well Separated Pair Decomposition(WSPD)
OK, I found it. The definition says 


  for any two distinct points ...


It means I must consider two points, not a one solitary. $x_1$ is well separated with $x_3$, and it is well separated with $x_2$ as well, But there is exactly one pair that separates $x_3$ from $x_1$ and also there is exactly one pair that separates $x_2$ from $x_1$