In Rhino Windows when exporting to an STL, there’s usually an additional detailed dialog box that asks about density, angles and ratios. Haven’t checked what Rhino OSX defaults to, but you might want to try zeroing everything out as shown in this post
http://discourse.mcneel.com/t/exporting-stl-mesh-files-from-rhino/10873/3
I used to get the same wonky meshes when Rhino 4.0 had those fields populated with something else.