A simple box section was modeled in the Section Wizard's Section Builder mode. The section properties reported by the program do not match with manual calculations. I.e. a moment of inertia calculated by hand is 1x10^6 mm^4, however, Section Wizard reports it as 1.827x10^6 mm^4. Why?
The hand calculation which is provided in the above screenshot are based on the thin wall theory (more details can be found here: https://structx.com/Shape_Formulas_025.html). However, the Section Builder mode in the Section Wizard is using a thick wall theory to calculate section's properties (more details can be found here: https://structx.com/Shape_Formulas_023.html).
Based on the online calculators, the thin wall theory provides such moments of inertia for the mentioned section:
-However, the thick wall theory in the same online calculator gives the same sectional properties as reported in the Section Wizard: