Because the surface of the rack in a selective or not is not smooth WH4 vertical tilt to one side makes the VNA and the cause, steel wheel, scratch off with guiderail.
Due to the surface during the formation of the selective rack in WH4 uneven or non-flat plane, makes VNA awry. And causes the steel wheels scraping against the guiderail.
Because the surface between the rows of the selective rack in WH4 regular or not is the plane, VNA tilt forward, and causes the wheel to scratch. Guiderail.