My husband is building me a raised bed vegetable garden (google square foot gardening if you're intersted). Anyway ... if he uses 8 foot boards and builds it 4 feet by 4 feet we have 16 square feet. If he uses the SAME materials, but builds it 3 feet by 5 feet, we have 15 square feet. Am I just having a blond moment? Why does it work that way, why do we lose a foot even though we are using the same materials??