The ignition system on our cars is known as a waste fire ignition. Two cylinders fire off the same coil at the same time. One of the cylinders would be igniting the power stroke and the other cylinder would be in the exhaust stroke (where the spark does nothing). A little later the other cylinder would be starting the power stroke and vice versa.
When the same coil fires two cylinders at the same time, one of the cylinders is fired in reverse polarity. Another way of saying this is one bank of the engine fires the plugs conventionally from the center wire to the side wire while the other bank fire in reverse from the side wire to the center wire.
The original plugs are a single platinum tip plug with a fine wire center wire of platinum while the side wire is steel. There is less electrode erosion when it is fired from the platinum tip. The cylinders that fired from the steel side wire will wear faster.
Earlier in Ford's history of waste fire ignitions, the original plugs had a platinum tip on the electrode that was fired from, so that there were two different styles of plugs in the engine. The specified replacement plugs were double platinum with platinum buttons on each electrode to make them "idiot proof" so that they would not be accidently installed incorrectly. Ford did a good job of educating their technicians on the importance of using double platinum plugs on such engines. Without explanation they changed to single plat fine wire plugs in 1996. Because of this early education, I still use double plats in my engine, either Motorcraft AWSF 32PP (the plug cataloged for 1995 Duratecs) or Autolite APP764 (which is the same plug with a different label, Autolite is the vendor that makes nearly all of Ford's Motorcraft plugs).
So the short answer to your question is that the rear bank of the engine is reverse fired and the side wire will erode faster than the front bank.
A solution would be to rotate the plugs from front to rear and rear to front half way through their life. A more sensable solution would be to just use double plat plugs in the first place.
