Constant VSWR circles are actually constant gain circles. Device itself has gain that coresponds to Gmax at a given frequency. By chosing the gain smaller that Gmax (selecting a constant gain circle instead of a dot that corespond to Gmax) you are mismatching the device. The mismach is constant on a constant gain circle, so is the VSWR. You can derive a formula to know exactly what VSWR is on your chosen constant gain circle.