QTR-8RC sensors array

I use QTR-8RC sensors array in a line tracking project, I use QTRsensors library.
The problem is when i started to track a black line the "position" values varies form 0 to 7000 linearly

BUT, when i started to track a white line over black area, the values did not vary linearly, they went from 3500 to 2000 then to 3400 etc.

I have already set the variable "WhiteLine" to be true

Thanks in advance !