Comment 4 for bug 1480414

The discrepancy is due to the fact that Inkscape presents two different types of reports, one of which is a visual box which includes the effect of stroke width, and one is a geometric box which does not include stroke width. In your case, if you hit the Select tool, F1, the size will be reported as 100 x 20 mm, which is the visual size. If you hit the rectangle tool , F4, the size will be reported as 99.746 x 19.746 mm. This is the geometric size, which is what the dxf file will see. The difference is the stroke width, which is .254 mm, which can be seen in the Fill & Stroke dialog, under Stroke Style.