Gun angles. 

i will take a stab at it....forty five degree ships...use he, and stay at max range......from twenty seven to thirty six use ap......anything under twenty seven use he since nobody armors belt.....for more info look up the guide on armor....

same shells hit,but different damage?

I guess it's because the shells hit different parts of armor.
As we know,wihte  figures mean  you hit the top armor，and green ones mean  you hit the side armor.
So when you hit  opponents with AP, big damage means you hit the top,small means you hit the side.