When I use 'Section Properties' to calculate the polar moment of inertia (i.e. the object's ability to resist torsion) of a shape that is NOT circular the sold gives me an answer the doesn't mach the textbook calculation.

For example when I calculate the MOI of a square (10x10mm) the solid says it's 1666.6mm^4 and according to the textbooks it's 10^4 X 0.141 = 1410mm^4

Why does it happens and how can see the object's ability to resist torsion?