That is the tympanum dome, a vestigial organ suspected of being a sonar receptor in their evolutionary precursors. The size, shape and texture vary among races and individuals. The blue-skinned Ukri are described as having "an over large forehead tympanum dome" on page 96 of Planetstorm. There are not very many pics that detail the face and skull. Each artist made their own variations on the theme. We don't, as yet, have a visual guide to go along with "The world according to Draegor and the genetic scientists of the Party" found in the margin of page 92.
Perhaps chronic steroid use causes distortions in the dome's proportions. Calvin Klein there is seriously cut for a Fantasian with their natural, subdermal insulating layer.