Zekov and Zul are bosses just likes The Defense of Eonar and Aggramar, there’s no sense in giving all stars rankings to these two bosses. Huge DPS difference just depends on lot of useless damage on mobs in the fight,such as totally using arane explosion in Zul fight or getting two Corruptor’s Pact in one fight and spawning Guardian of Yogg-Saron to deal meaningless damage at the end of Zekov fight.
I have got several all stars #1 in the spec I was playing and TOP 10 in all specs in past rankings, I don’t think it reflects player’s real gaming level, but makes the experience of pushing all stars rankings being really unfair, since you get a very good rankings as you can deal tons of useless damage on the trash on these two bosses.