All potential bonuses

it can't turn into 6% dex and 6% dex? Is it the same as the lines on a item that if the 2nd line is secondary it can't turn into a primary stat?

It's random, you only have about a 1/50 chance of getting a primary stat on the 2nd/3rd lines. So for every time you get 6%/6% dex, you'll get 6%/4% 50 times.