There are some tricks to tattooing that you only understand if you've been in the game for a while. Most tattooers know that the area of the body you're tattooing changes the way you tattoo, including the angle at which you hold your machine.
The general rule of thumb is that holding your tattoo machine at an angle gives you a greater chance of good ink saturation. Conversely, the more perpendicular you come at the skin, the less ink saturation you'll end up with.