8/3 = A2
6/3 = Detachable Carry Handle
0-5-10 = Match
The A2 rear sight is adjustable up to 800m, assuming you're using M855 from a 20" rifle. The elevation for 800m is twice as high as for 600, but the longer screw will just fit in an A2 upper.
In order for the elevation screw to fit in a detachable handle, it can only be long enough for 600m of elevation. Instead of using the same 8/3 wheel and having the wheel only go half-way around (stopping at the 600m position), they doubled the thread pitch, and you get twice as many clicks between 300m and 600m.
Someone later did the same thing (doubled the thread pitch, which gives you twice the number of clicks between elevation distances) for the A2 setup, to make a match sight that had finer adjustments. Since it was recognized that match shooters don't shoot M855, there wasn't much point in marking M855-related numbers on the wheel. Instead, there are simply markings to help count adjustment clicks.
The actual marked wheel is a separate part from the threaded half of the elevation drum, so it is possible to use the wrong wheel. Some AR parts companies only stock the 8/3 wheel and put it on A3 sights, even though the markings won't be anywhere near accurate. But, hey, it saves them a buck...