Hello!
Most likely “Disable double opt-in” feature is unchecked in your MailChimp subscribe settings.
Actions shows total number of actions with popup window – subscribes actions, likes, shares. It displays all numbers of users – which have confirmed the subscription and not confirmed.
In your MailChimp account you can see only those users, which have confirmed the subscription. Therefore you see such difference between subscribers in MailChimp and Actions in popup.