Hi,
The option EX0 is an option that applies for the fitting of TGraphErrors , TGraphAsymmErrors and TGraph2Derrors.
It means that the errors are set to zero for all the coordinates. Coordinate error does not mean point error. Coordinates are considered the independent variables (the features of the regression problem), which are X in the 1-dim case (TGraphErrors) and X and Y in the 2D case (TGraph2DErrors) and they are not to be confused with the dependent variables (Y in 1D case and Z in the 2d case).
Now option W instead set all errors to 1 and basically ignore all point errors. It is like fitting a TGraph instead of a TGraphErrors
Lorenzo