FIrst of all you have to make sure both item is of same element. from your picture you are comparing a ice element and arcane element. it will greatly affects the dps outcome because dps calculation factors in elemental DMG. and the types of affix you have on that weapon.
The legend item is strong if you are using secondary skill. skills like throw sword, earthshatter, whirlwind
The eternal item will stronger if you are using the primary skill slash and standing still. where dps is (Damage per sec)
its has 99k DPS because it has a 4 extra attack chance.
So if you are standing still dealing dmg. the eternal WINS legends. likewise the legend sword has much BETTER RAW power so it benefits spell caster.
im gonna give you a explanation with picture to help you understand better
Looking at the above picture you will notice the dps of the middle pic is slightly higher because im comparing the SAME element.
So if i will you i will ask the same question. WHY is my eternal item 75% lesser than my normal item. is is 75k dps!!
But take a back step and see the affix… because the eternal item is short of +225% Crit DMG
DPS calculation is very complicated but just remember 1 thing… skill are base off WEAPON dmg. what do we mean by weapon dmg. look at the above picture… the dmg on (B) is what the system use to calculate the damage of skills. like 500% of weapon damage ,800% weapon dmg. and so on… they take (B)
the affix that can affect (B) to go higher or lower are only WEAPON DMG and WEAPON DMG %
So What is (A)? A is the overall calculation of the affixes you have on that weapon including things like +Attack speed , + multi attack. Elemental DMG, Elemental DMG %… and so on…
Elemental DMG and and Elemental DMG % , Attack speed and + Multi attack , extra attack chance WILL NOT affect (B). it will only affect (A)
So at the end of the day the answer to my pic is. my mythic item is stronger than the eternal weapon.
Ok i will end it here and i hoped it will be a help to you… when you play more of the game you will understand its system and the deep item customisation that Dungeon Quest has to offer…