The coefficient is in response to your +damage. So if you have 1000 + damage to fire spells, fireball should get 652 extra damage on the cast and 348 extra on the DoT (I believe that becomes 87 extra damage per tick). So on fireball spam, instead of doing an average of 815+21 (for one tick of DoT) on the hit, you do 1467+108.
However, with the so-called "damage tax", you get only 587 extra instant and 313 extra DoT damage, or 78 per tick. SO you hit 1402+99. As you can see, if you dont spam fireball but you let the DoT tick (assume you put another spell in the middle, such as fireblast or refreshing scorch) you get more out of the plus damage.
Of course, if the coeffecient works different for fireball, I just wasted time on useless math lol.