The lesser nodes of the skill trial the harder it get.

For example Kakuzu skill trial only the double attack and +1 target standard attack is hard to get,because both of them only have 1 nodes for the skillbreak

,then you have the mastery and chakra scaling where both of them have alot of nodes so the chances of getting 1 is easier compared to 1 node skillbreak and the chase is stuck between middle stand of easy and hard to get.