The spline interpolation is done after the points have been mapped into the paintdevice coordinate system: the motivation is usually to make its representation smoother. This explains, why there is an impact of the current axes.

If you want to interpolate the data in plot coordinates you would have to use the spline classes manually and pass the result as curve points.
If you believe that the result of the interpolation is wrong you would have to give more information - f.e which type of spline you are using and - of course - a series of points, that goes wrong.

HTH,
Uwe