In the page referred to the setup of the RockBLOCK Satellite Modem, it's described that
1 RockBLOCK credit is used per MAVLink message. So, for example, if the RockBLOCK was configured to send 1 HIGH_LATENCY2 message every 30 seconds that would give a data usage of 60 credits per hour.
Ah hour contains 60 minutes or 3600 seconds, so the math actually results in 120 credits per hour (3600s/30s). That's the issue.