It’s hard to make a diagnosis without seeing the fit and the full covariance matrix values.
What do you mean by “to fix some values”? Did you make a fit with all your 9 parameters floating, and then fixed some of the parameters to the best fit values? In this case, it is indeed surprising that the fit result for the mean is so different.
Could you please provide the numeric fit results for each fit (with covariance matrix if possible) and tell us to which values you fixed your parameters? If you don’t mind sharing it, seeing the plot would also help!
Cheers,
Jonas