The physics behind the selectivity for the: O adsorption position on dihydride Si(100) surfaces is studied using HREELS and first-principles calculation. It is known that on dihydride Si(100) surfaces below 300 degrees C the formed Si-O-Si bonds do not jointly own any surface Si atoms when the O coverage is smaller than 80% of the number of outermost surface Si atoms. According to the present study, the selectivity is not governed by the electronic states near the Fermi energy at all. Instead, it is governed only by the energetics, and can be explained in terms of the strain induced by the adsorption itself.