An offset between the two coils opens up the sounds - scoops the mids a little
An offset is somewhere about 150-300 turns difference between the two coils.
In theory it makes the pickup a little less perfectly hum cancelling (not that many would notice) and makes the sound slightly more single coil like (and I mean just by a bit - but there does seem to be an extension of frequencies up and down.
The net result is a less middly, stodgy sounding pickup - some feel a little more character.
Often neck pickups have a greater offset to allow a less wooly sound.
In the past a number of companies offset their "humbucking style" pickups to get around the Patent that Gibson had on the design.
