When running a Manage Campaign process and the activity is 'Everyone Sent', Chimpegration processes what Mailchimp's API will give it. When Chimpegration asks Mailchimp for 'Everyone Sent', Mailchimp will provide a list of everyone that was sent the email in the campaign, excluding certain types of 'Hard Bounces'.
Mailchimp defines 'Hard Bounces' in a number of ways and as such, some 'Hard Bounces' will be includes in the 'Everyone Sent ' list it provides to Chimpegration. In most cases, Hard Bounces are defines as email addresses being 'incorrect' (for example, missing @ symbols) and as such do not exist. These are likely to be excludes from 'Everyone Sent' as they are identified straight away when email campaigns are sent out.
In other cases, 'Hard Bounces' can be soft bounces (for example email accounts that become temporarily or permanently inactive; full, etc.) that become 'Hard Bounces'. This transition from Soft to Hard Bounces takes a bit longer for Mailchimp to determine, so might be included in the 'Everyone Sent' list it provides to Chimpegration.
It is worthwhile highlighting that Chimpegration does not decide or influence how Mailchimp selects and provides lists it send to Chimpegration. Chimpegration simply works within Mailchimp's API framework.