Email Group Only Pulls subset of total emails
Reasons that come to mind for bulk emails not being sent to certain contacts:


Contacts with duplicate email address (only one contact will receive the email unless you uncheck the "Dedupe by email" option next to the Recipients box).
Contact has no email address which is marked either Primary or Bulk-Mailings and is not marked On-Hold.
Either of the following privacy settings: 


Do not email
NO BULK EMAILS (User Opt Out)