I think it's just that they are plotting sum vs ratio of two random variables. Try this (in R):
a <- runif(1000); b <- runif(1000); plot(a + b, log(a/b))
Also, they likely have a low-end cutoff (notice their x axis starts at 10^4. If you do the same to the above plot, you get even closer to that exact shape. Try:
a <- runif(1000); b <- runif(1000); plot(a + b, log(a/b))
Also, they likely have a low-end cutoff (notice their x axis starts at 10^4. If you do the same to the above plot, you get even closer to that exact shape. Try:
plot(a + b, log(a/b), xlim=quantile(a + b, probs=c(0.2, 1)))