When I subtract two histograms, the number of entries in the subtracted histogram is not equal to the difference in the number of entries between the two histograms. It seems that the resulting histogram does not contain the contents of the underflow and overflow bins.

This is not, however, the case for addition, which seems to preserve the sums just fine. Is this behavior expected? It is very unintuitive.

Step-by-step demonstration:

Get `TFile`

and `TTree`

and draw a branch into histograms (with different cuts):

```
root [0] TFile f("data/Datacut_noLcMcut.root")
(TFile &) Name: data/Datacut_noLcMcut.root Title:
root [1] TTree * t = (TTree*) f.Get("DecayTree")
(TTree *) 0x7fb9ae249060
root [2] TH1D * h1 = new TH1D("h1", "h1", 225, 4510, 5410)
(TH1D *) 0x7fb9ae170240
root [3] TH1D * h2 = new TH1D("h2", "h2", 225, 4510, 5410)
(TH1D *) 0x7fb9ae16fc30
root [4] t->Draw("X_LOKI_MASS_ConstrLc>>h1", "abs(Lc1_M - 2286.46) < 14", "goff")
(long long) 1253
root [5] t->Draw("X_LOKI_MASS_ConstrLc>>h2", "abs(Lc2_M - 2286.46) > 14", "goff")
(long long) 868
root [6] h1->GetEntries()
(double) 1253.0000
root [7] h2->GetEntries()
(double) 868.00000
```

Notice above that the histograms each have the same number of entries as the tree has that pass their cuts.

Now, if I subtract the two histograms, the resulting number of entries is not equal to the difference between the two:

```
root [12] TH1D * h3 = (TH1D*)h1->Clone("h3")
(TH1D *) 0x7fb9ae1b0970
root [13] h3->Add(h2, -1)
(bool) true
root [14] h3->GetEntries()
(double) 130.00000
root [15] h1->GetEntries() - h2->GetEntries()
(double) 385.00000
```

But if I add them, it is:

```
root [17] TH1D * h4 = (TH1D*)h1->Clone("h4")
(TH1D *) 0x7fb9ae1b2600
root [18] h4->Add(h2, +1)
(bool) true
root [19] h4->GetEntries()
(double) 2121.0000
root [20] h1->GetEntries() + h2->GetEntries()
(double) 2121.0000
```

Now, if I compare the number of entries in the subtracted histogram to the number of entries that don’t fall into underflow or overflow, I see that they are equal:

```
root [14] h3->GetEntries()
(double) 130.00000
root [16] t->GetEntries("abs(Lc1_M - 2286.46) < 14 && X_LOKI_MASS_ConstrLc>=4510 && X_LOKI_MASS_ConstrLc<5410") - t->GetEntries("abs(Lc2_M - 2286.46) > 14 && X_LOKI_MASS_ConstrLc>=4510 && X_LOKI_MASS_ConstrLc<5410")
(long long) 130
```

This is not true for the added histograms:

```
root [19] h4->GetEntries()
(double) 2121.0000
root [21] t->GetEntries("abs(Lc1_M - 2286.46) < 14 && X_LOKI_MASS_ConstrLc>=4510 && X_LOKI_MASS_ConstrLc<5410") + t->GetEntries("abs(Lc2_M - 2286.46) > 14 && X_LOKI_MASS_ConstrLc>=4510 && X_LOKI_MASS_ConstrLc<5410")
(long long) 1034
```

Is there a reason the behavior for addition and subtraction vis-a-vis underflow and overflow bins differs?

*ROOT Version:* 6.15/01

*Platform:* macOS Mojave

*Compiler:* Not Provided