Advertisers using similar audiences lists don't need to exclude people from original remarketing lists. Why is this the case?

Similar audiences are built from a seed remarketing list to help advertisers reach new people who resemble the original list rather than the original list members themselves. Google describes this audience expansion as a way to find new customers from existing first-party audience sources. In current Google Ads terminology, the same principle appears in Lookalike segments, which automatically exclude users from the seed lists. Because the original list is not included in the expansion audience, no separate exclusion is needed