The first MacBook gave Apple some headaches. One still remembers the yellowing of the plastic where the hands rested and the pieces of plastic that unglued themselves.
After some time they agreed to change these top-covers under guarantee, and one could believe that all had been regulated long ago. This is obviously not the case since the company informed its Service Centers that the references of the white covers of white MacBook (2006 and 2007) were going to change, which means that the new parts, although completely compatible, will be manufactured differently. Since there is this information concerning breakages at the edges, these new parts are thus there to prevent this problem in the future.