Single coils are already F-spaced by default. "F-spaced" means either Fender-spaced or Floyd-spaced, depending on whom you talk to.
Humbucking pickups were designed and for years used exclusively only on Gibson guitars, which have narrower string spacing than Fender's offerings. Fender designed their single-coil pickups for their own guitars' string spacing.
When guitarists started installing humbuckers in Strat-style guitars, it became apparent that the magnetic field of Gibson-spaced humbuckers wouldn't completely cover the string width of a Fender-style bridge/neck.
Most players just lived with it, but Edward
Van Halen compensated for the narrower spacing by rotating the pickup on a slight angle in order to widen the spread of the magnetic field by extending the "corners" out slightly. Years later, DiMarzio brought out their F-spaced humbuckers and Duncan started making Trembuckers.
