All the subscriptions on a server have the same list of user accounts in Subscriptions > exmaple.com > Users. Why does it happen?
These users are called Additional user accounts, as they do not belong to subscription itself, but to customer subscription owner. During the migration, those Additional user accounts were assigned to admin. And since admin owns all the subscriptions, those accounts are visible within every subscription.
More information regarding Additional User Accounts is available in our documentation: Additional User Accounts.