Phase cancellation as has been said. I THINK also that the distance between the coils determines (at least in part) the frequency of the scoop.
In a humbucker when the coils are side by side you lose a bit of the highs. On a typical jazz bass you lose some mid-mids (maybe 500-700hz I'm guessing). On a bass where one pickup is really close to the neck and one is really close to the bridge I believe that what you get is a lower mids scoop (say 300-500 hz).
The scoop generally sounds pleasing on bass but you have to be careful not to scoop too much in a live setting. Using both pickups and then also scooping with the amp can cause the bass tone to lose definition. I usually solo one pickup when playing live.
Would love to see the scoop frequency proved. I have tried to use the various freeware out there but can't figure it out. Just going by ear with my triple pickup basses.
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