This discrepancy can occur because of two reasons:

• Acoustic Campaign does not have a push token id in the contact record for a mobile device, so the message was not sent to the push provider.
• Acoustic Campaign did not yet update the contact's mobile record with the data from the Android or iOS Feedback Services Report.

For example, the contact source count in the Sent Message summary can show that the notification/message was sent to 23 contacts, but the report shows that the mobile message was only sent to three devices.