It is possible that the number of messages sent in Message Analytics does not correspond to the number of messages that you see in the dashboard of Email To SMS, SMS Mailer or Message API. That is because they are not the same metric.
Message Analytics counts the number of parts, while the dashboard in your message sending application of choice (Email To SMS, SMS Mailer or Message API), counts the number of messages.
For example, if you sent out 10 messages with SMS Mailer, of each 200 characters, the result would be:
Message Analytics: 20
SMS Mailer Dashboard: 10
In this case, you will be billed for 20 messages (parts).