Disordered thoughts about this:
missiles and weapon add to your strenght to indicate the damages done in one hit. So you may have a greater strenght, and if the other has better added damages than you, he may hit harder.
Some pet's habilities increase the damages done, like "get the stick" that increase missiles damages per 10%. at hight level, it's more efficient.
In case of critical hit, there's a multiplication. the more critical hit landing, the more damage done. But it's luck that decided if you get the critical hit in. Even when you have 50% chances, it doesn't mean you'll get one critical hit out of 2 attacks. you may get lucky and get 4 critical hit out 5, or unlucky and get one critical out of 10, since it's a new "throw" each attack.
And sometimes, an "All green" or "All red" isn't indication enough. if you're all green by only a few points, the critical hits and dodging chances are about evened. if the other has a greater luck (and/or a pet that allows to throw back damages/attack twice in a row) you may not win the fight.
In another playata game where the fighting system is very similar, players had noted that usually, out of 3 fights against one same player, there is an easy win, a hardest one, and one where luck is clearly on the other player's side. So maybe there's the same system at play here, and if you're not very superior, then you loose the "third".
In case of critical hit, there's a multiplication. the more critical hit landing, the more damage done.