It's a variable that's tied to the health and damage scaling of encounters, which dont take into account the impact certain spells will have on the player, or the impact that certain player spells will have on the enemy.
Lancet hits harder than Rubim and has more health, but Rubim has spells that can cc you,and you have an ability that essentially counters Lancet's more physical focused fighting style.
the fighting mechanics are still being fleshed out, which makes Rubim quite strong in the current version.