GeomAPI_PointsToBSpline not working properly

Hello All,
I used GeomAPI_PointsToBSpline to generate a bspline curve. when I use the high tolerances such as 0.5 it seems that the output is not within the specified tolerance. I attached the pic of output which is plotted in MS Excel shows a deviation much more than 0.5. Any idea would be appreciated.

by the way the z coordinate of all data is zero and the number of Scatter points is 100.

regards,

*****The code:
GeomAPI_PointsToBSpline BSCurveAPI(Scatterpoints, 5, 5, GeomAbs_C2, 0.5);
Handle_Geom_BSplineCurve BSCurve = BSCurveAPI.Curve ();