Three::Draw integral depends on number bins and weights

Hello,

I am using TTree::Draw to make histograms and I found that the integral of the histogram that is produced changes depending on the number of bins in the histogram. Here is an example of what I see:

root [38] nominal->Draw("mtw>>hist1(1,0,200e3)","3.21e3*weight_btag_77*weight_pileup*weight_mc*weight_lept_eff/AMI")
(Long64_t) 3698629
root [39] nominal->Draw("mtw>>hist10(10,0,200e3)","3.21e3*weight_btag_77*weight_pileup*weight_mc*weight_lept_eff/AMI")
(Long64_t) 3698629
root [40] nominal->Draw("mtw>>hist100(100,0,200e3)","3.21e3*weight_btag_77*weight_pileup*weight_mc*weight_lept_eff/AMI")
(Long64_t) 3698629
root [41] cout<<hist1->Integral()<<endl;
588.132
root [42] cout<<hist10->Integral()<<endl;
589.498
root [43] cout<<hist100->Integral()<<endl;
589.464

What is even stranger is that the relative behavior changes if I change the weights. Here I include the additional weights XSection*FilterEff*KFactor, all of which are the same value for all events:

root [44] nominal->Draw("mtw>>hb1(1,0,200e3)","3.21e3*weight_btag_77*weight_pileup*weight_mc*weight_lept_eff*XSection*FilterEff*KFactor/AMI")
(Long64_t) 3698629
root [45] nominal->Draw("mtw>>hb10(10,0,200e3)","3.21e3*weight_btag_77*weight_pileup*weight_mc*weight_lept_eff*XSection*FilterEff*KFactor/AMI")
(Long64_t) 3698629
root [46] nominal->Draw("mtw>>hb100(100,0,200e3)","3.21e3*weight_btag_77*weight_pileup*weight_mc*weight_lept_eff*XSection*FilterEff*KFactor/AMI")
(Long64_t) 3698629
root [47] cout<<hb1->Integral()<<endl;
266867
root [48] cout<<hb10->Integral()<<endl;
266213
root [49] cout<<hb100->Integral()<<endl;
266240

Any idea what is happening and how I can fix it so that I get the correct normalization for every case?

Thanks.
~Joe