Filling two RH1D histograms and adding them using ROOT::Experimental::Add( h1, h2 ) results in the wrong number of entries. The reason is the way the histograms are merged:
It is using ApplyXC which fills the bin content to the target histograms. This is wrong, as the bin currently being filled may have been filled via multiple Fill() calls. So the final number of entries after merging is n original entries + nbins from the "from" histogram.