All relays*, well at least all the ones I know about, have a lower drop-out voltage than closing voltage.
This is due to the magnetic circuit within the relay.
When the relay is in the resting state, there is an airgap in the magnetic circuit, and it takes a greater magnetising force to move the armature of the relay.
When the relay is closed, there is no air gap in the magnetic circuit and it takes less magentizing force to hold the armature in the closed position.
JimB
* Afterthought.
While writing this, I am sure I remember some relays which did not completely close the magnetic circuit and so would "drop out" quicker when the voltage was removed.