The damage per turn works like other DoTs in that they benefit greatly from attack power. Maxed Skadi has 696 attack, while yours is at 548. Once your Skadi is level 80 the DoT will be the same, because they will have the same attack power. From the DoT guide:
Why does my DoT not match the hero card at the summons gate?
Occasionally, people report that their DoT hero is bugged as their special skill delivers less damage than advertised. This in fact is not a bug, but a feature pertaining to the evolution of heroes. As an example, below is the hero card for Colen (who afflicts “Burn damage”) as found in the summons gate.
For heroes that feature DoT, the amount of damage is a function of their base attack stat. In other words, the total damage inflicted will be less than that shown on the “advertised card” until the hero is fully leveled and the special skill is at 8/8. Below is an example of a partially leveled Colen.
With the introduction of emblems, it is possible to increase the DoT above that shown on the “advertised card.” This will occur as base attack stats are increased with emblems. Note that if you choose to emblemed the defensive path of the hero, you will not see a change to DoT. Below is an example of an emblemed Colen who has been buffed offensively.