The contacts on a lists can be reduced when contacts unsubscribe, contacts choose “Do Not Email”, emails bounce (depending if it is a hard or soft bounce), multiple emails to a contact are marked as spam, and email addresses/domains have errors.
The following knowledge base article explains the difference between hard and soft bounces, as well as errors:
Sending messages to the same list multiple times per day (or 24 hour period) can substantially increase the likelihood that remote mail servers will reject emails and cause the emails to bounce.