TF1 GetParameter : accuracy is limited

[code]// …
#include
#include
// …
std::cout << std::setprecision(17)
<< "Parameter(2) is " << fitFcn->GetParameter(2)
<< std::endl;

std::cout << std::setprecision(7)
<< "Parameter(2) is " << fitFcn->GetParameter(2)
<< std::endl;
// …[/code]