[OR SS4] Fillet curve transitions - offset method doesn't seem to be correct

I'm working on reverse-engineering some standard drawings so that I can repurpose them for nonstandard conditions. This leads me to making sure I have everything plotted correctly. This leads me to analyzing the attributes of the transitions and all that.

So, here I am, looking at a back deflection given on the standard drawing as 8d 32m 29.65s. The radius is 2890'. The length is 430.837'. The tangent is 215.818'.

When I draw up a curve to represent this, I set the back transition as follows: type: Curve; method: Deflection; radius: 2890'; deflection: 8°32'29.65".

I change the method to length. The length field displays 430.8369'. Great.

From my calculations (and confirming with an offset line), the lateral displacement of the PT should be offset 32.05483' from the back tangent.

When I change the method to offset, the offset field displays 16.4156'. What does this 16.4156' represent? If I manually set it to 32.05483' as it should be, I get a much wider curve, as you might expect.