The 2 curves in Test_GCon.3dm (31.1 KB) overlap.
_Distance in V5, V6, and WIP reports 0.0047 between their closest endpoints.
_GCon, on the other hand, reports these endpoints to be:
V5
Curve end difference = 0.0047 millimeters
Radius of curvature difference = 5.7530 millimeters
Curvature direction difference in degrees = 0.0255
Tangent difference in degrees = 0.0255
V6 & WIP
Curve end difference = 0.0001 millimeters
Radius of curvature difference = 5.7494 millimeters
Curvature direction difference in degrees = 0.0140
Tangent difference in degrees = 0.0140
I have a RhinoCommon script that (in all 3 Rhino versions) reports the V5 results.