Here are some most common reasons why the number of sent messages in your audience campaign may be different from the estimated segment reach.
In case you are sending a multi message campaign, please make sure that you retrieve the segment size per each message in Messages tab - not via calculating the whole segment in Target group tab. Using sub segments may reduce the overall reach of your campaign as some users may not be satisfying any of the conditions you applied in the sub segments.
It usually takes some time to get response data from your channel provider and interpret it in CrossEngage dashboard. If your campaign has just finished dispatching, please check back some time later to ensure the statistical data is accurate.
Take a look at the warnings in the Recent Activity dashboard: bounces and attempts to send messages to corrupt email addresses may reduce the number of valid sends.
Check whether global frequency capping (Settings -> Cappings) might affect the reach.
Consider that the size of your database is permanently growing so comparing the current segment size with the number of delivered messages days or weeks after the dispatch can also result in discrepancies in numbers.
If none from the above appears to be relevant, log into the account at your channel provider and see if the number of sent messages there is equal to to the number of triggered messages in CrossEngage. If it's not, consider checking the discrepancies on the side of your channel provider or talk to their Customer Support.
Should the discrepancies in number still be not interpretable, please submit a request.