So I changed my spark plugs about 1600 miles ago. when i removed my original plugs that came with the car new, they were completely charred/burnt halfway down the threads. these new ones are the NGK laser iridiums as well.

after i did the change, my MPG went back up to about 22MPG for about 2 tanks, and then fell back down to abouit 19.5. so after a few weeks i decided to take them out and check them again and here they are. its only charred/burnt on the top as you can see. Are they ok? or is something wrong with my engine where it is shortening the life of the plugs?