Calibration - Layer Height

To continue the RepRap calibration process (yes you really need that much calibration) I printed an object to verify the Layer height.
The object printed perfectly but I noticed my wall thickness is 0.75mm instead of 0.40mm. Something that needs some investigation. I also verified that the X and Y axis calibration is perfect now; 20mm in any software gives an object of exactly 20mm!

Object: 4-mm-wall.stl by muelli from Thingiverse
Hotend: 245°C 1st layer, then 240°C
Heatbed: 110°C
Material: 3mm ABS (Silver from RepRapUniverse.com)
Layer height: 0.2mm
Wall thickness: 0.4mm
Infill: 100%

Static props: 0.4mm nozzle, E3D All metal hotend, MK2a heatbed, Gcode generation with Slic3r, Pronterface, Sanguinololu, Ice Blue Stepsticks, NEMA17 motors, Prusa i3, Achatz Edition, ATX powered 400W