yeah that's possibility of Nurbs, set of same points does not means the same curve. Arc or circle, if I'm correct (I could be totally wrong), this is the case when nurbs curve is 'rational', parts of NURBS curve are affected by multiple points.
In some DCC apps, like Maya or Softimage, it's possible to change the way how curve is drawn, relative to control points- but not to make the perfect circle or arc ( again if I'm correct). In MoI, formula is chosen by method you used for drawing.