ZBrushCentral

shadowbox extruded sized (z-depth) thinner in middle than edge sides

(Brush 4R8)

When I use ShadowBox, I find that the middle of the extruded sides are much thinner than at the edges. At least this is what ShapeWays is telling me when I try to upload my models to them. Their tools are complaining that my walls are much thinner in the middle. Although, measuring in ZBrush they come coming out to about 2mm. However, Shapeways thinks the centers are more sub 1mm.

My shadowbox settings that I am using are 720/8.

Any ideas what is causing this and how, I can fix it?

I have attached an image showing what I am talking about.ZB ProjectionIssue.jpg