Hi Walter,
Because of De-duplication of the UV.
There will be visitors who will be using different search engines to reach your websites. So in Search engines report they will shows as different UV under different line items ( for e.g. Google and Bing) but when it comes to the referrer type reports it will de-duplicate the same visitor from the total who is using different search engines to reach you.
Let me know if it helps you. You can mark it as closed if it solves your query.
Thanks
Akhil