Q u o t e: So then it isn't possible to design a system like so?
x < y < z
x% IAS - An attack is 20 frames
y% IAS - The same attack is sometimes 20 frames, and sometimes 19 frames
z% IAS - The same attack is 19 frames
What is it that prevents this?
Item randomization. Equality of item % bonus prevents flavor in the items. You would need to statistically calculate the best from the worst items to coordinate from the lowest to the highest frames. Which is possible, however significantly more complicated, not to mention the human eye would not be able to tell the difference between frames depending on how many are imbeded. It's a video game, and with that kind of work, you would probably have to pay 2 to 300$ for that kind of design feature that you would never truly see.