i am wondering why i get a dufference in a cosmetic thread vs a thread cutout.

i make a 3/16 dia round bar i go to thread and i can do a 10-32 thread np. but i saw a utube video saying it takes up alot of memory from the computer .fine i can understand that. so i see i can use a cosmestic thread. now when i het the edge where i want it to start. i can only get up to a 8-32 thread? why? and what am i doing wrong? i use ansi std.. but if i choose none i get a thread but how do i know what size? since i just says cosmetic thread?