I got sent a link to a 'Ford' forum where they were discussing Zetec plugs.
As we know, the Ignition system on the Zetec engine is 'Wasted Spark', which means that two plugs are always firing (aka 'wasted spark') on the exhaust stroke.
With this approach, two plugs fire through the centre electrode while the other two fire through the side electrode.
So, to save a bit of money, Ford made plugs with the platinum tip on the centre electrode on some, and others with the platinum tip on the outside electrode.
Obviously, these plugs have to be fitted in the correct cylinders.
With replacement plugs, all of which are double platinum, you can fit them anywhere.