About why we do it reversed: Imagine you have two important packages, an accounting package and a graphics package. If you specified these in ‘straight’ order:
accounting.mycompany.org
graphics.mycompany.org
Then it implies there is a major accounting package, a subsection of which is for mycompany, and a subsection of that package is called the org package which you actually use. However, you want this:
org.mycompany.accounting
org.mycompany.graphics
This makes more sense. Out of all packages from organizations (org), you look at mycompany in particular, and it has two sub-packages, the accounting and the graphics ones.