When designing a plan under SW15 in welded constructions mode, I noticed that the sketches take into account the width of the profiles. Let me explain: taking the example of a rectangular table of 800 x 1600 mm with square tube profiles of 40x40x2 mm, well the initial sketch must imperatively have a dimension of 760 x 1560 to have a correct list of materials in order to have the dimension of 800 x 1600 again, so I'm talking to you pros to know if it's logical or I'm the one who is not only a NOVICE, but maybe I need to adapt settings in this software...?
Looks at the position of the profile in relation to the sketch (centered, interior, exterior). In your case, the profile must be centered on the sketch so half a width of the profile is "lost" on each side
Here is a link to the help: http://help.solidworks.com/2015/french/SolidWorks/sldworks/c_Weldments_Pierce_Points.htm (at the very bottom: modification of meeting points)
If necessary, put a screen view, I could guide you.