Leads are counted when someone fills out a form that they arrived at through a promo tool. Referrals are counted when the first or last referrer field is set with a Partner's information. In many cases leads = referrals.
However, there are at least two common conditions that will result in a mismatch in the number of leads vs referrals:
1. Referrals can be set manually, by a rule, or through a Partner link. These referrals will not show as leads, as no form was filled out.