Damage calculator problem?

[Lianne]

My pet has 86 strength and when fighting the inflatable balthazar using a stone club and a golden compass (so, 18 icons) and fierce attack I do 81 damage and yet the damage calculator says I should do 63. I know it can be a bit random but not that much surely! My Neo username is Tekhi if you need to get in touch. I wanted to work out what damage I would do with beserk attack for when I get to level 50. Thanks.