Since BrightTALK is an always-on community of business professionals searching for and watching content, you may find that your content continues to see new viewers come in during the live presentation or in the weeks and months after it.
If these viewers did not register/pre-register before the date of the content, they will only be counted as a viewer and not as a registration. In other words, the registration number is effectively frozen once the content goes live.
This leads to a common situation where the number of viewers is actually larger than the number of registrations. This is actually a good thing- it means more people have viewed your content than not.
Note: On-demand only content, such as video uploads, will only ever have "Viewers" for this same reason.