Sunburn can happen anywhere the sunlight meets the leaf at a 90 degree right angle. Which as you mentioned Lucy, typically is in the center of the leaf. The edges tend to deflect slightly downward lessening that 90 degree right angle. The sunlight reflects off of the leaf surface causing less heat buildup and less possibility of a burn.
Don't ask me how I remember that?! I know my head is just full of random things like that.
