User blog comment:SayuriDarling/The House of Hades Official Discussion/@comment-24308873-20131020014711/@comment-3146930-20131020165940

However think about their realms of power by total area. The sky for example is Zeus' realm and covers the entire earth. However unlike Poseidon or Hades, his realm would have the smallest amount of subjects (like birds, gryphons, ect.). Poseidon's realm covers most of the planet and as we all know, the ocean is absolutely filled with various lifeforms (both real animals and mythological ones). And lastly the Underworld is filled with the dead and his children have some control over the earth.

Now taking all this into account. Even if children of Hades and Poseidon overall have more powers, children of Zeus are able to use their powers almost anywhere because the sky is almost anywhere. Percy for example can call water to him, but this is tiring and he could lose control (BotL). However as Jason put it, there isn't an ocean in Kansas. Jason could control winds or cause lightning, but Percy in that location couldn't fight at full power. Same could be said with children of Hades. They can call the dead from the Underworld or control precious metals, but if they are out at sea or flying, they are almost powerless. Even if a child of Zeus was Underground, they wouldn't be able to call down lightning, but they could still control the wind.