What could be the reason for not seeing a user in a segment when using the User Preview tool?

The rationale behind the choice that the user preview tool only shows a random subset of segment users is that this feature is designed to help users quickly assess the makeup of segments without overwhelming them with too much information at once. The tool selectively displays a limited number of users from the segment being analyzed; thus, if a user does not appear, it may simply be because they are not part of the random sample shown at that time.

Additionally, it's important to note that the user preview tool does not guarantee visibility for every user within a defined segment because it randomizes the display, which means that even if a user is indeed part of the segment, they might not appear in every preview session. This approach allows marketers to get a glimpse of representative users from a larger segment while managing the performance and usability of the tool.