Tim,
Sorry, I did not have access to a mic when creating this video. I have attached a .vce file for reference.
The baseline in the example has a greater line weight in the center of the two offset lines. If you explore both offset lines, using the baseline as a reference line, you'll notice that they are sloping when in fact the should be sloping at 0% due to the fact that I offset them vertically at a value of zero. It seems to give incorrect elevations on every attempt unless you offset the entire linestring and break the line where you need it, making the offset segment function useless if you have VPIs on the linestring.