Because its basically hinge paint rubbing off the cover and onto the body of the iPad.. looks JUST LIKE A SCRATCH on the iPad but it comes off easily.. I have seen 5 different smart covers (3 buddies, 1 i bought on 3.11, and the relpacement) .. they all do the same thing.. i wouldnt bother getting an exchange unless you want a different color than what you originally got.. your new cover will be doing this too within the first day...
before anyone jumps down my throat...
this is just adviced based on MY EXPERIENCE with 5 smart covers... im not saying ALL covers are doing this... just the ones i have come in contact with...
here is a thread i made about it earlier..
https://forums.macrumors.com/threads/1115080/