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If you want the outside face of the wheel to stay in the same spot when you go to a wider or thinner wheel, the offset must change to keep the mounting interface at the same place with the hub face.
Pretend this is the top down view of your front left wheel and you want to keep the outside (left) edge of either wheel flush with the fender.
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On the thinner wheel the offset is 0 (middle), but to keep that front left wheel at the same outer position, the wider wheel needs to move the offset away from the middle.
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