So you have a number of unregistered Products and you want to get people to Activate their Registration, so you send a registration reminder campaign.
The strange part is, the recipient count of the campaign is far less than you expected.
Let us explain why:
Registration Reminders are sent to the Billing email address of any Order, that contains an unregistered Product. And because an Order may contain more than one Product (i.e. more than one registration), you can't directly compare the unregistered Product count to the registration reminder campaign recipient count.
See how to create registration reminder campaigns here.