You dont quite understand the maths as well. Take a die and roll it 100 times to see if you can roll four of a same number in a row, theres a good chance you wont. the chances of rolling the same number 4 times in a row will be a smaller than 1/6. I understand the % doesn't change from roll to roll, but its easy to understand rolling 3 in a row will be harder than 2 in a row and so on. Now if it does a random roll for what piece of armor, then a roll on what item you would have a 1/4 chance roll then a 1/2 chance roll.