In segment Cross section area there are three options (row, column, and slice). Since we always work with 3D data in Slicer, all of them will generate a result, even in a single slice. Perhaps you are not choosing the right axis.
If you are unsure which axis to choose, create a single circular segment with the diameter of 40mm (paint segment tool). The cross-sectional area reported for that should be about 1,256mm2. Go through all three of them and see in which one you see the correct cross-sectional value, then repeat the same with your data.