Just starting with a target number from 1-30 is a lot clearer odds when you know you’re rolling a d20. What’s more, reducing the difficulty by 1 (and thus the target number by 3) generally costs… 3 points from an attribute pool. Why are we not just directly subtracting those points from the target number? If they still wanted to restrict it to batches of 3, well, ok, but still, directly applying it to the target number seems a lot more straight forward.