trace width showing incorrect value when provided with layer thickness and impedance value

There is lot more than the dielectric thickness and over all PCB thickness the distance to the nearest reference plane loss tangent etc etc 

The Cross-section Editor uses an internal 2D Field Solver, which requires a plane / shape to act as a reference for the traces to calculate impedance.

• If no reference plane is found, the 2D Field Solver will place a reference plane outside of the PCB outline to act as a reference and in most cases, impedance will rise because the reference plane is far away. The impedance value reported is good enough for users to know the effect of the lack of a reference plane.

• Most 2-layer designs have no dedicated reference plane, so impedance is controlled by referencing the side of signal traces only, but the Internal 2D solver cannot solve Coplanar structures.