I have a segment that is made from people being on my Newsletter list and list from an event. The number of people in the segment is smaller than the combination of the two lists. Are people getting skipped?
If a person belongs in both lists, they will be added to the segment once instead of twice. Find the total number of people who are in both lists (you can use segmentation to do this), and see if that accounts for the missing numbers in the segment. Check out our article on Segment Conditions for more information.