(New Shopbot owner and new to CNC. Background in design.)

I am trying to get my CNC to make an accurate profile cut, but can't figure out what is wrong whether it is geometry or software settings or machine setup.

I took the images below which tell some of the story.
http://imgur.com/a/IQpIg

Basically the problem is....
18.0 mm in rhino = 16.14 mm when CNC'd
18.5 mm in rhino = 16.32 mm when CNC'd
19.0 mm in rhino = 16.56 mm when CNC'd
20.0 mm in rhino = 17.35 mm when CNC'd

Also I need to cut 20mm down to get guarantee cutting through the 18mm plywood of which the cut into spoil board is less than 1mm deep.