That is because the only HD diff ratio is 3.29. It's all in the drive/driven gears. For links on all this, please search on my name and "trans info".

Basics are the HD trans comes with 37/33 drive/driven aka 1:1.121 combined with 3.29's this works out to 2.93 overall ratio.
Since the sprockets always count 70 teeth total, drive chains are always the same length and the splines are always the same. You can mix and match, to get a 3.29 ratio, you put drive/driven of 35/35 from an NA/not HD trans, multiply 1:1 by 3.29 diff and you have 3.29 overall.