DH Shadow Build - Impale setting lock?

[ShinKiba]

Why is it that when I try to set the DH Cluster Size setting to 1 it automatically set the Cast Range to 10 but if I set the Cast Range to 100 it automatically adjust the Cluster size to 15???

How do I make it so that it will Impale everything at max range no matter if it's a single enemy or group???