Open plus not open totals don't add up to total sent

I'm using the Contact "create segment" page to find my least engaged contacts, but I'm confused because the numbers are not making sense to me. When I query who has received an email in the past year, the total returned is 5,270. When I query who has received an email in the past year and opened an email in the past year, the total is 4,172 When I query to has received an email in the past year and has not opened an email in the past year, the total is 714. Should total opened plus total not opened equal the number who received an email? Here, the total who have opened plus the total how have not opened is 4,886. These leaves 384 contacts who are reported as having received an email but are not in their the opened or not opened segment! Can anyone explain this to me? Any help in this will be greatly appreciated!