Segments show how many users in the system match the conditions set in the segment, but that does not automatically mean that they are available to receive messages.
There are many reasons why the dispatches don't match the segment size.
User can be excluded, because they:
- are opted out or blacklisted
- were excluded because of frequency cappings
- were excluded because of exclusion rules
- are in a Control Group (campaigns)
Usually these reasons have their own specific event that will show up in the event history of the users, e.g.: Message Dispatch Failed Because Of Control Group