Umbrella sampling bootstrapping

GROMACS version:
GROMACS modification: Yes/No
Here post your question : I have performed umbrella sampling for ligand pulling. For the first time it gave me a histo and pmf plot. Than I performed it again for bootstraping with smiliar configuration/sampling windows. Both the graphs are different. How is it possible that I am getting two different histo and PMF graphs with same conf/sampling windows?