Difference in getparrerror()and sqr of cov matrix element

Hi. I have a related problem. I am fitting a simple pol2 function ( [0]*pow(x,2)+[1]*x) to different TgraphErrors with only errors in the y axis.

In some cases (not in all my tgraphserrors), there is a difference between the uncertainties obtained for one of the fit parameters, when I use the method getparerror(0) or (1) and when I calculate the sqr of the covariance matrix element (0,0) or (1,1). Also the uncertainty in this cases is bigger when I use the getparrerror() method. And the uncertainty is bigger than the parameter value.

When this happened for one parameter, for the other the uncertainties match with both methods and the values are much smaller than the parameter value as it should be for a useful fit.

I want to know which method gives me the correct value for the parameter error in these cases. And I want to know if the covariance matrix element (1,0) is well estimated also when this happens.