Much of Lucchese's great fit is owing to the perfected lasting process. In it, the upper is secured to the sole by hand. The upper is wet with water, pulled over the last, and tacked securely. Once dry, the Lemonwood pegs are inserted. It's a very physical process and one more detail that sets Lucchese boots apart.
Also see "A Study in Detail, Part 1: Lemonwood Pegs," and "A Study in Detail, Part 2: Hand-Stitching."