There are several possible explanations for this phenomenon. First, it is possible that the band gap in doped graphene is an effect of the doping concentration. As the doping concentration increases, the band gap could become more pronounced. Second, it is possible that the band gap is a result of interactions between the dopant and the graphene lattice. It is well known that dopants can modify the electronic structure of materials, and this could lead to a band gap in doped graphene. Finally, it is also possible that the band gap is a result of quantum effects, such as electron-electron interactions or the formation of bound states. Ultimately, further investigation is needed to determine the exact mechanism behind the band gap in doped graphene.