Not seeing the brake, but based on the model, I gather that you're talking about a recessed nut.

If so the nut probably can't go farther because you're bottoming against the material pushed forward when they broached (pushed a hex peg into a round hole) the hex .

You see the same type of reduced size hole when you look at socket wrenches.

In any case, you need only about 5 threads to be engaged to achieve maximum strength, so if the nut turns 5 turns or more you're fine. The key is to use a nut of the right length to meet that 5 turns goal on your fork.

BTW- the threads on your front brake are overly long because they have to provide for longer nuts and various thickness fork crowns. More thread than necessary on the bolt give you more latitude in matching nuts and forks.