Ah, yes, my misunderstanding. Though, with Ley Lines and Sky Islands, there’s something going on up higher. Maybe it’s more concentrated within those lines themselves, thus leaving the rest weaker.
(Experiments at seeing whether spells can be cast more efficaciously while riding a Mist Barge tend to end poorly due to the Mist Barge losing its power when its rerouted to a spell. Researchers – those who survive – continue their efforts.)