if you look at that picture, you'll see that teh threads are at teh top of the plug, there's about a half inch missing off the bottom, the plug goes down thru the head in an area that traps debris, the area below the threads get gummed up with carbon, if they had threaded the plug all the way, and made it a little thicker around the porcelain, there would be no issue, but they cheapened out, making the plug thin and not threaded all the way down
heres a little more info about it
http://www.agcoauto.com/content/news/p2_articleid/120
http://www.wcpo.com/dpp/news/local_n...-calls-routine