I printed a 20mm calibration cube and the x and y were very close to 20mm, whereas the z distance was 18.9mm - help!!!

When I print a 20mm calibration cube .....the Z direction is only 18.8mm instead of 20, whereas the x and y are very close to 20. Any ideas why this would be ..its repeatable....and the bottom of the cube is see through as the first layers are paper thin.
Thx
Tim