reference cam and actual position

hello dear friends.
The following statements is not my problem and i state it
only for my understanding.
we have a machine which has one axis controlled by
sew movidrive and motor.The axis go to reference position
after each power off and on. I command the axis to
reference travel and after traveling,it stop and hmi panel
display the actual position=880
i power off the machine and move reference cam to right about
1cm and then power on the machine. this time after reference
traveling the hmi display actual position=400
The reference traveling is such that the motor start to
rotate right to pass reference cam then it reverse to find
encoder zero mark then motor stop.
i am curious to know that why after moving reference cam
the actual position change?(however the encoder
has only one zero mark)
For your information in this machine the motor encoder turn
less that 1 revolution (less than 360deg)

thanks.

dear experts.
i am waiting for your reply!!!!
thanks.

We need more information. What units is the HMI displaying? What homing method is being used?