When you import contacts, we scan the email addresses and perform a validation. If the email address fails validation, we mark the contact as "Undeliverable". If you send and email and the email bounces, we mark the contact as "Undeliverable".
If a contact has an "Undeliverable" status, the system will not send emails to that contact. If you add contacts that have "Undeliverable" to an email campaign, they will not get added because they cannot be sent emails.
I checked your account and the missing contacts on the email campaign an you have around 600 contacts with undeliverable status which is why you are seeing 2100 on the email campaign.