Originally Posted by
Niminae
Yes, this is exactly the same issue which the Artificer abilities Shatter Defense, Thunder-Shock Weapon, and Mighty Slam all share. They have the same sort of DC calculation using Sunder Modifiers, and just as you note with Feral Charge and Improved Trip, those abilities seem like they should add +4 to the DC of Feral Charge, Shatter Defense, Thunder-Shock Weapon, and Mighty Slam. But the actual DC calculation given doesn't bear that out, since both Improved Trip and Improved Sunder just have a 4 higher base DC and use the same calculation for their bonus values.