How does "+Elemental Damage %" affect MH skill damage?

Take Whirlwind for example.

According to the codex description, Whirlwind does 500% “weapon damage” and weapon damage is described as “flat weapon damage”.

If Whirlwind dps is based on flat weapon damage, why would removing a “+20% fire damage %” reduce the damage of the whirlwind skill on the stat page? Is this because 80% of MH damage is added to whirlwind as fire?

So would it be more accurate to say the whirlwind skill damage formula (with a mh fire element and a +20% fire dmg affix) is:

( 500% * weapon damage over 3 sec ) +
( ( 80% MH damage over 2 sec ) * ( 1+ 20%) )

Can anyone help me understand this?