My mate has two settlements where defense far surpasses required for the amount of settlers yet still has a RED numbered stat next to "defense".
Explain Bethesda?

Doesn't have to do with the number of settlers, it has to do with the amount of resources you produce there. Tons of Food and Water? Tons of reasons for raiders to attack so bigger defenses are needed.

Defenders stat should equal the combined sum of your food and water.