You capture the transmitted data volume on your active network interface. If you compare these values to the ones displayed in the customer control panel, you will see different values.
Cause / Problem solving:
The traffic accounting (=Capturing of the data volume) is done at the network interface to which your server is connected (the respective switch port of the server).
The traffic display in the customer control panel is solely based on this data collection and may differ from other data collections (e.g. the one done by yourself). This difference is due to technical reasons.
This doesn't imply an error but is due to the location where the data is collected. The other perspective on the data volume resulting from this behaviour causes slightly different values displayed in the customer control panel.