Draw ves Project

Dear developpers

I have done the following
1) open a ROOT file with a TTree which contains "zp" and "pdg" as variables 2) do the tree drawing
tree->Draw("zp","pdg==211")
Then just to show for student an other possibility 3) create a TH1D* h1 with ("h1","z prod of pi+",100,0,100) 4) project the tree on h1 as followed
tree->Project(h1->GetName(),"zp","pdg==211") 5) draw the result
h1->Draw("SAME")

To my great surprise the 2 plots does not fit each other (see pb.ps attached), and this is not just bin to bin fluctuation Have you an idea?
Regards
Jean-Eric

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