my 03 did that as well with 4.7 not sure what caused it but never bothered to investigate it as it never seemed to cause any real issues or problems.

I did however put the ngk v power in the 4.7 at 170k miles and I swear up and down that the old plugs were the stock as you mentioned before mine were also no diode barely anything left though I waited till I got cel for misfire before I realized how bad they needed to be replaced. NGK I saw like a .2 increase (calcuated by hand) but my truck did feel much peppier which could be result of going from toasted plugs to new plugs regardless of brand.