By comparing the QI maps of a π-system (benzene) and a σ-system (butane), as well as the transmission phase of the orbital contributions, we have shown that the orbital contributions to constructive/destructive interference are quite different in these two cases. Namely, the transmission dip in the transmission for metasubstituted benzene arises due to phase differences between paired MOs. These contributions cancel exactly as the orbital pairing theorem guarantees that they are equidistant from 0eV and that the individual contributions to the transmission are of equal magnitude. In contrast, for σ-system of butane the transmission dip is due to cancelation between non-paired orbitals as the position and width of individual orbital contributions to the transmission changes with the dihedral angle changes. Together, these results provide a richer picture of interactions and contributions to destructive interference, and an indication of how orbitals might be manipulated to exert fine control of quantum interference effects in molecules.