The more data you plot the more you'll find an exponential increase in sales for every increase in Gross Ranking. And if you watch the App Store rankings that

The more data you plot the more you'll find an exponential increase in sales for every increase in Gross Ranking. And if you watch the App Store rankings that makes sense. The apps at the top stay there, in the same relative order, for long periods of time. This suggests larger gaps in sales, vs lower ranking apps whose rankings fluctuate constantly. The purpose of this exercise was to make reasonable estimates, not to get it exactly right.