Originally this was done to even out string response. String technology was a little different when those pickups were first made. Used to be that the G's were wound and they were a little louder than the other strings.
Modern strings are made to perform more evenly. Humbuckers never were staggered though you could adjust the poles.(Not that anyone does)
Today some builders, Lollar for one, tend to make single coils with a flat radius. The argument being that it is no longer necessary to stagger the poles and humbuckers work just fine flat so why not singles too.