How to evaluate curve fitting SSE and RMSE values for the same fit

Hi All, I have created two curve fitting fits with "X data" as "x", "Y data" as "y", "Z data" as "fxy" and "Custom Equation" (z=f(x,y) with the value

a+b*cos(pi*x)*cos(pi*y) +c*sin(pi*x)*cos(pi*y) +d*cos(pi*x)*sin(pi*y) +e*sin(pi*x)*sin(pi*y)

The results/"Goodness of fit:" for the both the fits are different though the equation is same.

Goodness of fit:1 SSE: 9.182e-028 R-square: 1 Adjusted R-square: 1

RMSE: 1.515e-016

 Goodness of fit:2
  SSE: 9.916e-028
  R-square: 1
  Adjusted R-square: 1
  RMSE: 1.575e-016

Why the SSE and RMSE values are different, is it expected result or how to get the best result.

Thanks in advance.

With regards, G.K.Jagatheswari.