I don't really understand leads in the campaigns tab on Zoho CRM

I don't really understand leads in the campaigns tab on Zoho CRM

Good morning, fellow CRM users. 

I am having quite a hard time understanding the leads in the campaigns tab. In this particular campaign in the screenshot below, we sent out emails to our mailing leads through Zoho Campaigns, and the data is linked back to CRM. 
Even though we had 38 recipiences who opened their emails, the system only registered 3 leads and one pre-existing contact. I'm quite confused as to how this happened. Can anyone help?

Thank you in advance!