Why did laban trick Jacob regarding his marriage?

In Genesis chapter 30, Jacob agreed with Laban that he would keep all the sheep that were brown, and all the goats that were speckled, with Laban to keep those of a pure colour. He then put magical rods in the watering troughs and held the rods before the eyes of the cattle before they mated, so that they would conceive speckled offspring. This seems to have been handed down from a more ancient creation story that told how speckled sheep and goats came to be.