The needles has different taper at the tip. Low speed has the longer tip and High speed has the shorter tip. Practically but unpractical, in a pinch it is possible to use needles with the same taper in both positions if they have the right lengths so they reach the needle seat. As the threads are same on both needles, when you turn the knob a certain amount, say 3 clicks, the different taper makes the measured amount of fluid pass a specific hole or orifice different. So with the longer taper you have a finer division of measurement than with the shorter tip. Of course the seats in the carb also are different, but as the needles never are turned all the way closed, it is only the amount of gas they are letting past that they regulate.
The picture is from my DLX 63. Tapers and lenghts may vary on different carb versions and brand.
