Hey There,
I think i've figured it out. when I removed my visitor-level segment, the Unique Visitor count by day was consistent, regardless of the date range i set. I think that the segment I was using created a visitor cohort that changed based on the specific set of days that I had created. The Visitor would only need to fulfill the criteria in my visitor-level segment within that time period, and they would be included in my report...that would explain why the # of visitors each day varies based on different time periods.
Hope i'm making sense! heh...
Mike