I have a cabinet back that I'm trying to notch out the back corner. In the cabinet and line drawing editor the back correctly measures 45 1/4". When I take it to the parts editor, it correctly lists it as 45 1/4" wide, but the piece itself that is showing is off by 3/8" of an inch. Now I know that I can get the correct location of my cutout by adding or subtracting that amount, but why is the part smaller than what all the dimensions are everywhere else?
To answer your reason why your off your 3/8" is because when you bring your back into part editor, it shifts your part over by your dado settings. Since you are using the standard settings of 3/8" depth for your dado on your back, this is why you are off by 3/8". This offset can easily be seen if you turn on your origin lines in the grid settings of part editor. (Accessible by using the icon or by right clicking. Once in grid settings, you will be able to turn on your Origin Axis. These are visible by the red lines on your grid.) When you have this on, you will see that the part overhangs the vertical line by your dado depth.