When printing an object such as a hollow sphere the vertical walls will be thicker than the horizontal ones due to the fact that there is less material per unit of surface area as the curvature increases. As you approach the top of the sphere, if you hold the extrusion rate constant, the amount of material that on the vertical sides built the thickness of the wall has to spread out and build the entire external surface. This causes the wall to become thinner and thinner as the surface approaches horizontal.
A helpful feature to include in Simplify3D would be to compensate for the curvature and hold a constant wall thickness throughout the object. This is something that is currently impossible.
