[Elecraft] K3EXREF question

[ANDY NEHAN]

Sometime ago I installed the K3EXREF which I feed from an external 10MHz double oven Xtal oscillator. I am confident the K3EXREF is installed and configured correctly as when in use and the external oscillator is connected the front panel says REF*CAL with the * blinking. Attempting to change the local oscillator frequency gives the message "Locked" just as the manual says.
However, I am puzzled as the local oscillator frequency display slowly drifts down from 49.379.675Hz to 49.379.622Hz over a period of at least an hour from K3 switch on.  I leave the external oscillator on permanently. I was expecting the local oscillator to rapidly get to its final frequency. Can someone explain whats happening as I am puzzled??
My firmware is the latest version.
Andy G4HUE