Wrong Pull for ROOMCSTUDY

Hi,

I am using RooMCStudy in this way

RooMCStudy mc_study(b,x,Binned(true),Verbose(false) );
mc_study.generateAndFit(2000,1000);

can1->cd();
RooPlot* frame_p2 = mc_study.plotPull(p2);
frame_p2->Draw();
can1->SaveAs("p2_pull.pdf");

can2->cd();
RooPlot* frame_p3 = mc_study.plotPull(p3);
frame_p3->Draw();
can2->SaveAs("p3_pull.pdf");

for a given pdf b, when I see the pull I sometimes see a good (around zero with width 1) pull and sometimes not. If I generate from b pseudo data and then I fit this to b, the pull should not always be good? What does it mean if its not? The function is a bad function or maybe there is a bug in RooMcStudy?