Isochrone basis

If I select a point, for example a centroid of a land area, where does the isochrone centrepoint get placed? Im particularly interested when dealing with walking and cycling. Sometimes I will have a large site and centroid is some distance from any road or path. However the isochrone (distance based) is often off centre and shows say 1000m much less than 1000m from my initial point.

Does this help ?