Bases for stress calculus (see Basics - Bases of calculus) do not consider the wire curvature.
The curvature of the wire induces and increase of stress in the inside part of the spring.
For a torsion spring, the most significant stress in the body of the spring is bending (for small helix angles). A correction factor is applied in order to consider the wire curvature.
Corrected stress related to torque M2 :
for DIN standards: q = (w + 0.07)/(w - 0.75)