Calculate 3D surface length from a DEM

[GraemeArmour]

Trying to calculate the 3D surface length of a polyline on a DEM. 

Using the Add surface information & interpolate shape tools both work but return almost identical number if not the same value. Over a 50km DEM with changing elevation it should be more than the 2D length. See attached image. Both 2D shape length and 3D Shape Length identical at 45.031Km

Seen a similar post on here but no solution as to why the value returns the same.