The default voltage curve is not an exact match for the amount of voltage that the CPU needs to run 100% stable. Offset voltage reduces voltage along the entire curve. An offset that is OK for full load stability will usually not be enough when cores are lightly loaded. Running 1 or 2 threads of Prime95 or similar is just as important as full load testing.
A laptop that is stable at full load and part load at -180 mV rarely happens.