Ballista explosion bugged? Half dmg when destroyed than when expired

I noticed something strang with ballista explosion.
Having Armed construction with frigid barrage.
When I place a ballista and it expires it does 100% more dmg (when this hit also chills) than just spamming ballistas on the ground and destroying the ones before because of ballista cap.
I have no idea why, expiring or destroyed by my own replacements should do same dmg?

anyone has an idea?

Did nobody notice that too?