Honestly, I completely ignore the tooltips in the “damage” section… They are nice for reference but they use a different calculation from the damage itself.
As for Combustion, I tested the numbers on my character the other day and found where the numbers comes from. I posted the details in the thread where people are discussing Combustion.
But now I’m thinking, if the tooltips in the “damage” section follows a different calculation than the damage itself, what guarantee us that the text from the skill tree actually reflects how the skill is being calculated. Maybe Endless Pyre and Combustion are working fine and the problem is just the text… But if this is true, then I have no idea how to test if the Skills/Passives/Node/Glyphs are work correctly.