Cloud Kicks recently sent a flash sale email to a data extension containing 1,300 subscribers.

In Salesforce Marketing Cloud, exclusion lists are used to suppress specific contacts from a send. If Cloud Kicks applied an exclusion list, it would prevent certain subscribers from receiving the email, which would explain why only 950 out of 1,300 subscribers received it. The exclusion list may contain subscribers who have opted out, unsubscribed, or were otherwise flagged to be excluded from this particular email.

Using Exclusion Lists: Exclusion lists are managed at the send time and are often used for regulatory compliance or to manage email fatigue.

Salesforce Documentation Reference: See Exclusion List Management for guidance on how exclusion lists impact email sends.