I understand what you mean, but that is not the case in all conversion types.
For instance when converting physical dmg to other dmg types.
But I guess this is special case cuz dmg that is going to be converted is counted as two types of dmg first as elemental and as 2nd as fire, or other way around.
So its more like: X to Y, Z, etc tallied together than A to B, C, etc tallied together.
And ofc that dmg cant be converted twice cuz that would make infinite loop in certain situation. For example when you have items that are converting physical to elemental and elemental to physical or so.