How do I get alc per optic sens for non alc sensitivity

So in recent times ive observed a number of good players and they have shown me that they use default (non alc sens) but one things has left in confusion, and its that for their per optic sens they are able to change and have decimals in them where as my per optic sensitivity is based off fixed numbers. As an example im only able to choose from 1,2,3,4,5,6,7,8 sensitivity for the 10 optics you are able to change to sens on, and they have their sensitivity set to 2.8 for 2x or 4.7 for 6x optics. I was wondering how im able to access this feature as well. as we both use non.
alc sensitivity