Once upon a time, all Fender Stratocasters arrived from the factory with a trio of more or less identical pickups. Sure, vintage production techniques meant there was a little more wiggle room for the number of winds on each pickup, but there was no inbuilt difference between a bridge pickup and a neck or middle one.
Up until 1977, Strats were shipped with a three-way pickup selector switch. Your only pickup selection options were bridge, middle or neck. Enterprizing players figured out that they could achieve combinations of bridge plus middle or neck plus middle pickups by balancing the selector switch midway between the two. (Seymour W Duncan mentions during an interview in Dave Hunter’s book The Guitar Pickup Handbook that in the early 70s one of his jobs at the Fender Sound House in London was modifying the three-position Centralab 1452 Stratocaster ‘knife’ switches into five-position ones).
Players and techs loved the unique sounds they could achieve by using these previously unavailable pickup combinations, and they soon realised that by winding the pickup wire in the opposite direction around a magnet with the opposite polarity to the other two pickups, the middle pickup could become hum-cancelling when combined with the other two pickups. This is the same basic concept behind a humbucking pickup, although the sound is quite different to a regular humbucker. There’s a noticeable dip in midrange and a little more ‘string sound.’
Most Seymour Duncan single coil pickups – including single coils for seven-string guitars – are available in reverse wind/reverse polarity versions for hum canceling in these “2″ and “4″ pickup selector positions. You won’t notice any difference in the sound when the middle pickup is used by itself, but as soon as you combine it with one of the other two coils you’ll hear the zippier, springier, airier and hum-free tone that you can’t achieve with three single coils of the same polarity of each other.
By the way, there’s an interesting tidbit in Tales From The Custom Shop related to this popular mod: a July 1980 order by Eric Johnson for some modification work on a trio of old Stratocaster pickups, including the reverse wind/reverse polarity mod for the middle pickup. See it here.