Why do paid campaigns seem to drive conversions that would happen anyway?

It feels like I’m paying for customers I would have gotten for free.

S: At 4.2%, the blended paid conversion rate is up eight-tenths of a point month over month. C: The mechanism is attribution window overlap. I: When cannibalization is excluded and spend is held against only the conversions it actually created, the true incremental CPA typically runs two to three times the reported blended