Display names are generated based off the settings in Site >> Settings >> Content.

General Applications fields will be available to map the desired Display Name configuration, in addition to the following rules.
  1. If no Display Name fields are selected: The most recent display name or primary Email Address will be selected.
  2. If Display Name fields are selected but the General Application Fields have not been completed: The display name will be the Applicant's Primary Email address.
  3. If the account has no Applicant Role: The display name defaults to the Primary Email address unless otherwise edited on the user's account. 
  4. If you are utilizing the Stewardship Module please see the article What is the difference between Display Name in Content Settings versus Display Name in Contact Imports? to understand display name mapping differences for contacts.