The V-I diagram is taken at steady state current (when measured current becomes steady after some time from switched on) in air of 25℃ ambient condition.

On a standard ambient / heat dissipation environment, the PTC’s steady state surface temperature is practically same. And the power (wattage) within a small variance even if different voltages are used.

This characteristic gives self-heating PTC the advantage of dual of multi-voltage usage in different countries of different voltage.