How can I validate the results of sftool (curve fitting) ?

I have set of position data (x,y,z) for a curve/trajectory in space . I want to find a formulation/equation for this curve by using sftool . I tried 4th degree polynomial to fit the curve and I got good results with proper goodness of fit.

But when I want to check/validate the obtained formulation/equation by substituting the values for x and y in the equation , I get a completely wrong values for F(x,y) which is different with my z value.I think my z values should be close to the f(x,y) values !!

Could you please help me with my question ?