Notices versus visitors
Notices are displayed to visitors who have not consented (positively or negatively) yet. Therefore, after a user makes a choice on the consent notice, they will not see the notice again for months (the time frame depends on your notice settings). Visitors and notices are two very distinct metrics.
Notices versus new visitors versus positive consents
As the analytics trackers should only be triggered once positive consent is given, the new visitors metric and positive consents metric should be very similar.
The volume of notices, however, takes into account the fact that the notice is going to be displayed multiple times until the consent request is answered. The average amount of times a banner will be displayed before a choice is made will be influenced by the notice format and the consent trigger.