Not all of the customers were migrated. Why is it so?
Some shopping carts allow to create the same e-mail addresses for different customers. It means that system does not block/stop the registration with the e-mail address that is already in use. As the consequence, users have the same e-mail addresses.
In such situation Cart2Cart will migrate only first customer and another will be skipped. That’s why quantity of customers on Source and Target stores may be different.