I suppressed a list and confirmed before sending that 4743 recipients would not receive an email. Yet, they did! What happened? How can I be sure that the marketing cloud tool is working when it says "excluded"?

Lets say that you have a data extension of 1000 records in that, you don't have to send/exclude 100 records in the same data extension.There are many ways to exclude :

• Suppression list
• Exclusion data extension
• Exclusion script
• Auto suppression list (This will exclude based on sender profile or Business units) 

To your question on suppression list, it is excluded based upon email address. If you the sender's email address is present in the suppression list then it would be excluded.