The Arikok National Park takes up approximately 18 percent of the island of Aruba and includes three primary geological formations: the Aruba lava formation, a quartz diorite formation, and a limestone formation that extends inward from the coast. These formations have directly influenced Aruba's human settlement, as well as its natural wonders.
The Aruba lava formation takes up 15 percent of the island of Aruba.
The text doesn't state what percentage of the island each formation takes up, only that they are all included in the Arikok National Park. Since the park takes up 18% of the island, any percentage lower than that is at least possible for any of the formations. The system probably thought it was incorrect because it was a different percent than the text stated.