Why did Harlow Shapley find that clusters near mid-plane of Milky Way were further away than those in halo?

Harlow Shapley, in the early 20th century, found that globular clusters near the mid-plane of the Milky Way disk seemed to be systematically further away than those in the halo. What was the problem here? What effect did Shapley fail to take into account, as this effect was not known at the time he did his research?