User blog comment:Hayashi H/Fall 2015 Fleet Setups/@comment-26874340-20151125120013/@comment-26486243-20151125182145

It's an RNG effect, as night battle in single fleets does not account for formation modifiers or engagement modifiers. Having insufficient damage to take out any target except the boss in day battle is a reason to use Line Ahead instead of Double Line, but having insufficient damage in night battle to finish off the boss is not something the engagement modifier or formation selection will affect. Just be sure to place the ships in order of ascending firepower so the first few ships can scratch while the strongest ones finish it off - scratching after the big guns have already fired does not change the battle result.

The reason for the use of 10cm+AAFD whenever possible over the 12.7Bk2 is the accuracy and evasion bonuses it gives (the AACI will only trigger if an Air Radar is equipped, so if you're using an Akizuki-class anywhere, be sure not to equip an Air Radar on the non-Akizuki classes so they cannot override an Akizuki API 1 AACI), but in terms of damage, they are identical at identical upgrade levels.