Discrepancy between # of contacts in smart group vs # of contacts in smart group mailing
I think we've worked it out! A while ago some contacts had been manually added to the Smart Group (their status on the contact list for screenshot 2 showed them as 'Added' rather than 'Smart'). Smart groups don't automatically remove these people, so I've manually removed them which should solve the problem. 

Going to avoid manually adding people to Smart Groups now and let them do their thing!