Pythia8 Hadronize the stable Dark matter particles

Dear Root and Pythia8 experts,

We are trying to analyze the channel related to charged higgs pair production pp–>H+H–>W+W-+2S, where S is the dark matter candidate. We are using Inert Doublet Model and running our code based on Pythia 8 to hadronize the LHE event file generated from Calchep have above mentioned process. Our problem is that we are receiving higher jet multiplicity than expected. Since we have 2 W bosons and we decay them fully hadronically, we are expecting jet multiplicity peak around 4 jets, but we are getting multiplicity peak at 7 to 8 jets/per event. Though at the level of Calchep we undecay ~X - the undecay dark matter candidate.
Please if some one can help us to figure out the mentioned problem

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