When you're building projects, it's a smart idea to pick up about 20% more than called for in the materials list. There are several good reasons:
First, boards aren't perfect. You may have to work around some defects like splits, knots, etc.
Second, as much as we want to be, we aren't perfect either. We're bound to make mistakes from time to time, which is frustrating enough. Having to make an unexpected run for more materials just makes it even worse.
Third, having extra lets you test and experiment. If you've never tried a technique before, you need to set up a tool, you want to test stain colors, or you just want to practice an operation, you'll appreciate having some extra material on hand.