The Default Account Expiry field indicates the number of days that must elapse without a successful login before users in the selected community will have their accounts set to Non-active (and therefore cannot log in). This would be beneficial for security reasons.
The Account Deactivation field indicates that user accounts in a community will be set to Non-active (and therefore cannot log in) after a specified number of days regardless if the users have logged in successfully or not. This would be beneficial in the case of temporary or contract employees.