Reed hysteresis exists between the pull-in value and the cut-off value, which is usually expressed as the ratio of DO/PI expressed in %. Hysteresis can change with different designs of the reed switch (see the figure below), the thickness of the plating or sputtering, the hardness of the dry reed, the overlap of the dry reed, the length of the dry reed, and the length of the dry reed. The size of the gap between the reeds, the length of the glass tube, etc. will all affect this parameter.
The above figure shows the range of pull-in and cut-off values. Note that this hysteresis change is very small at low AT (AT).
And as the value of AT increases, the change will increase.