An offset curve to an ellipse doesn't form a mathematical ellipse. In Inventor or in AutoCAD you can either create a true equidistant curve to an ellipse (with constant offset) - i.e. a spline, or an "offset" ellipse, i.e. a true ellipse with non-uniform offset along its circumference.
In Inventor you can choose between these two types of offsets in the context menu of the Offset command (O) - the default mode "Constrain Offset" creates true offset ellipse. If you disable the "Constrain Offset" mode after you select the source ellipse, Inventor will create an equidistant curve, a spline.
Example: Offset to the inner ellipse - the red one is a spline (disabled constrain):
In AutoCAD, the OFFSET command creates an equidistant curve, i.e. a spline. If you want to make a derived ellipse, use e.g. the LISP utility "OffsetEllipse.lsp" - see OFEL (true offset ellipse).