I recently acquired Marjana and Sartana and got both to skill lvl 8, and I noticed something strange. The skill power for mine is weaker than the skill power for the ones you can preview in the epic summon. The % dmg is correct, but the DoT dmg is significantly weaker.
Damage over time effects are based not just on skill level but also hero level, because they’re based on the attack stat. When you max them the stats will match.
See for example:
Oh, okay. Weird. Did not know that! Thank you.