It's fairly common to see slightly different numbers for a single message between the Message composer and the Analytics dashboard. This is because of how those "counters" are calculated, and how often the data is refreshed.
Number of users is the average users per day that fit the test's targeting criteria (based on an average of DAU over the past 3 days). Because it's based on an average, it's possible to see slightly more or slightly fewer actual users in your test.
Number of users is based on the subset of your user base that fits the targeting criteria, at the time the message is being composed.
New Lifecycle Campaign
Number of users is the average users per day that fit the test's targeting criteria (based on past data).
In the Message composer: Number of users is based on sent messages in real time.
User counts are not in real time and refresh every 2 to 4 hours, so can be slightly lower than the real-time actuals seen in the campaign view.