Hi Matt,
I'm actually trying to create the model of a real part I physically measured - so I don't have anything to section.
Hopefully this picture will explain the issue:
I created the datum points by the "offset coordinate system" method, which allows me to easily import the coordinates as a .PTS text file.
Then the blue curve is what I get if I use the "datum curve through points".
The red curve is what I get if I sketch a spline on a separate sketch through the individual points (its last point coinciding with its start point).