This happens for to several reasons:
- The device opened a data session but did not upload/download any data (usually occurs when there is short session duration).
- The device suddenly closed the session (power off, etc.), then the session is still active in the network, but is shown as zero-byte session as no data is going through the open session.
- The device is opening several sessions at the time, but as data can flow only through one open session, others will show as zero-byte sessions.