Board Thread:Questions and Answers/@comment-30538704-20191229223519/@comment-26464279-20191230141541

That is about the overkill protection mechanic that is likely to trigger for anything with low armour and HP like DD, CL, AO, etc.

On the right is the percentage chart for the outcome if overkill protection is triggered and here is also a link with the formula on the combat page.

https://kancolle.fandom.com/wiki/Combat#/Damage_Calculation#Player_Ship_Protection_Mechanisms