Why does the return factor of the current relay be less than 1?

Relay coil through a certain size of the current, it will move, when the current is called the action current or start the current. Then, if the current is reduced, when the current is reduced to a certain extent, the relay returns, when the current is called the return current. Obviously, if the current continues to increase after the operating current, the relay will not return. It can be seen that the return current is always less than the operating current. And the return coefficient of the relay, is the return of the current and the ratio of starting current ah. Therefore, the return factor of the relay is always less than 1.