Reason for variance between Subscribed list and Sent (when sent to all) ?

Good afternoon, 

Our contacts indicate we have 3600 subscribers, when I send an email campaign report indicates 3023 were sent.

I wanted to identify the 577 that were NOT sent. Thought I would export full subscribers - 3600 - and then export the Sent, but when I click the all on the Sent report it jumps up to the full total. 

The full list 3600 shows all active - this is a week after clearing bounce and suspended lists - no flags. 

Send indicates it is send 99.7% yet total sent  

From the Campaign Report: 3023 Sent

 When I attempt to create list of Sent, and click the box to select all records, the item count increases to 3,602

What would be the variance? 

I appreciate any suggestions. 

Thank you