I created two separate segments, one where a custom event exists at the Visit level and one that exists at the Visitor level. When I apply each of those segments to their respective metric (Visits & Unique Visitors), the Unique Visitors count is higher. How is that possible? Shouldn't the event count at the Visit level be greater than the Unique Visitor count?
I encountered a similar behavior: Unique Visitors on a Segment with "Event X exists" at a Visitor level are much higher than pure "Event X" counts!.
If I find the solution, I will tell you.