The onset of frictional motion for elastic sliders with extended rough surfaces is governed by the rupture dynamics of the interfacial contacts. In these systems significant variations of static friction coefficient have been found for the same materials. Here we propose a model for understanding how and why the values of static friction coefficient can vary within wide limits. We establish a relationship between measured values of static friction and preslip stress profiles at the frictional interface and predict how the range of variation of the static friction depends on material properties and on the size of the slider.