Talk:Summer 2019 Event/@comment-27132740-20190918050745/@comment-26082284-20190918060639

When the game determines attack outcome it includes a random number in the calculations, so the result vary and same ship sometimes can crit, and sometimes fail completly - this is armor roll.

For details see kancolle.fandom.com/wiki/Combat/Damage_Calculation (armor roll is random number in Defense Power formula).