A robin's eyes sit on the sides of its head, so it cannot look straight down with both at once, and the hop, freeze and head-cock brings one eye to bear on a patch of soil. The same movement also turns an ear toward the ground, and the two readings of the posture are not mutually exclusive.
Controlled aviary work separated the cues. Robins offered mealworms buried out of sight still found them with visual, olfactory and vibrotactile cues excluded, and their success dropped when white noise masked the sound of prey moving in the substrate. The earlier experimental study had concluded that robins forage by sight alone; the later result shows hearing is sufficient on its own for buried prey, and surface-visible prey is plainly taken by eye.
The wider misreading concerns the robin's status as a sign of spring. Many American robins do not leave northern latitudes at all. They stop defending lawn territories, form flocks and switch to fruit, so they become far less visible without going anywhere. The bird reappearing on the lawn in March is often announcing a change of diet, not a return trip.