Xbcdr40su ls for press brake problem

Hello

i have PLC from LS XBCDR40SU , I used it for press brake , as i used rotary incoder to define the positions .

now , if i put two measuring points as for example : 20 mm , 30 mm

the first time is : 20 mm , 30 mm

second time is : 21 mm , 31 mm !!!!!!!

third time is : 22 mm , 32 mm !!!!!!

and so on uncrease one mm for each bend , why ?? and how can i solve it ??

thanks