I added some theory and a drawing in the blog to this (towards the end) which I was unable to do earlier, because my monitor refused to present me with an image.
The airbrush on the left is held perpendicular relative to the surface and the one on the right is held at an angle. Blue indicates the more or less solid part of the spray and red stands for the overspray. Below the surface you see the top view of the paint footprint. What you notice is that in the perpendicular situation the mixture (of paint water and air) tends to escape in all directions indicated by the purple arrows, making the footprint lager. In the angled situation the mixture escapes in a narrower fashion, nearing a more unilateral flow, indicated by the green arrows. In addition the part of the mixture that hits the surface first, bounces off the surface and interferes with the part of the mixture in the spray cone that is still traveling towards the surface, causing some mini turbulence resulting in less paint reaching the surface than I suppose is the case in the perpendicular situation.
Of course this is all assumption, an attempt to figure out why it works for me (and the likes of outlandishly talented artists like Alberto Ponno).
