To generate vehicle envelopes in AutoTURN, the vehicle is placed every certain distance (step), and the envelope is generated accordingly. Hence, the step length (100mm or 4 inches) influences the precision of the envelope (2-3 mm or 0.1 inches difference depending on the vehicle).
This tolerance was deemed as acceptable for the profile envelopes (making them slightly more conservative), since an increase in the precision has also a direct impact in the performance.