Visitor Count disagrees with webpage registration

We run provisioned webcast events where visitor register. They are allowed to the event through this registration which gives us (normally) a good count of viewers. I understand that often multiple visitors to one of our programs comes from an IP address that is shared by many users. I expect, normally, the Piwik/Matomo user count to be lower due to this software “seeing” the viewing IP (hope I’m right here). We’ve used the system for a few years. This week, for the first time, the opposite occurred. Our system showed approximately 400 viewers, Piwik reported 800. Any ideas on how this could happen?