I'm using Advanced Queries to look at data on Uniques, but total Uniques site-wide are less than the summed total when I add a Group parameter to the query. Why?
As a general rule, Uniques cannot be added together (summed from a segmented query). We determine unique users by a distinct cookie ID for each user/device accessing your website. A single unique cookie ID will only be counted once in a site-wide (unsegmented) query, but as soon as a Group is added to segment your unique user count, each unique cookie ID can be counted in multiple rows of your segmented query. So adding together your segmented uniques totals will always result in a higher amount than total uniques site-wide from your unsegmented query.