Savage Blow's top and bottom hits do same damage

It has to do something with the adventurer rings: When I have my cruel ring equipped, every 5th blow is the same as the first

 Spoiler

Crit also:

without my cruel ring, it's the same with the sixth.

 Spoiler

It has to do with the criticals (not the chance to 1hitko) because when I unquip my ring and equip my lv 30 durability dagger which gives 7% criticals, it's also the 5th.