You are not missing anything. What you want to to do is not an option with the chamfer commands in Rhino. But here is a method to do it.
ConstantWidthChamferDC1.3dm (274.4 KB)
Create a mid-surface between the two surfaces to be chamfered using TweenSurfaces
Offset the mid-surface one-half the width of the desired chamfer on either side using OffsetSrf. The Flip option in OffsetSrf is used for one side.
Check the intersection of the offset surfaces with the original surfaces. If they do not intersect the full length of the original surfaces then either extend the offset surfaces or go back and extend the mid-surface and then offset the extended mid–surface.
Trim the original surfaces using the offset surfaces.
Loft the trimmed edges to create the chamfer.
Join as desired.