ACCURACY LIMITS with fitting with TGraphs

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Dear experts,
I am trying to fit a TGraph with with 2 parameters.

 **********
 ** 1780 **MIGRAD        5000     1.6e-05
 **********
 MIGRAD MINIMIZATION HAS CONVERGED.
 MIGRAD WILL VERIFY CONVERGENCE AND ERROR MATRIX.
 MACHINE ACCURACY LIMITS FURTHER IMPROVEMENT.
 MIGRAD MINIMIZATION HAS CONVERGED.
 FCN=3.81108e+09 FROM MIGRAD    STATUS=CONVERGED      58 CALLS          59 TOTAL
                     EDM=1.09335e-07    STRATEGY= 1      ERROR MATRIX ACCURATE 
  EXT PARAMETER                                   STEP         FIRST   
  NO.   NAME      VALUE            ERROR          SIZE      DERIVATIVE 
   1  Dead time    4.24292e-07   1.71668e-13  -0.00000e+00   2.38551e+09
   2  Efficiency   9.01102e-01   8.69533e-07  -0.00000e+00   5.98989e+02
 FCN=3.81108e+09 FROM MIGRAD    STATUS=CONVERGED      58 CALLS          59 TOTAL
                     EDM=1.09335e-07    STRATEGY= 1      ERROR MATRIX ACCURATE 
  EXT PARAMETER                                   STEP         FIRST   
  NO.   NAME      VALUE            ERROR          SIZE      DERIVATIVE 
   1  Dead time    4.24292e-07   1.71668e-13  -0.00000e+00   2.38551e+09
   2  Efficiency   9.01102e-01   8.69533e-07  -0.00000e+00   5.98989e+02
tdead 0.000000 efficiency 0.901102

What does is the line below saying:

MACHINE ACCURACY LIMITS FURTHER IMPROVEMENT.

and how come i am not able to extract the estimated value for parameter[0] = 4.24292e-07 as it returns tdead 0.000000 with

tdead = fitfunc->GetParameter(0);//say fitfunc is of class TF1, and SetParLimits (...) is not invoked

F.