You measured from the 1" mark ("burning" the first inch), then forgot to add an inch when marking. Solution: First, use a steel rule or folding rule that doesn't have the play of a retractable tape's hook. Second, if you must burn that inch, make your mark, top, then double-check it by measuring again using the hook, bottom. This example shows how burning that inch resulted in greater accuracy, as the bottom measurement is 1⁄64" shorter than the tape indicates.