because your hook will hit in the middle, between 2 and 3 wire, then drag to the 3 wire
and you can get an OK and not get a 3wire, and you can also get a 3wire and not get a OK (or a fair or a no grade,etc)
PS: they, as in the LSO's, can change the aimpoint though by changing the Fresnel lens, like when the 4 wire is busted, they'll target the 2 wire instead
They want you to aim for the middle so that you have the best chance of catching a wire, if you're alittle high, you still catch the four, if you're alittle low you'll catch the one or two but still safely away from the ramp. If they were targetting the one for example, and you were low, well, ramp strike. Targetting the four, and your high, bolter. Targetting the hook point there is the best for safely bringing you aboard.