The status changes when the user connects to the Internet. If there were no attempts to connect to the Internet, then the status will remain the same.
There is a page called Viber Support where you can see everything about the software and the way it works: https://support.viber.com/
However, there have been numerous inquiries that the online status of Viber is wrongly reported to users. This could also be the cause to the wrong status report.