Random isn't exactly random. The Chronivac searches for potential subjects within its 150 foot range, so depending on where the Chronivac is located and how many people are in that circle, the selection may not be that random. If Jeff hadn't been alone in the house, and most of his neighbor's been at work, he might not have ended up where he did. Neither Jeff's father nor brothers were home. The Chronivac automatically excluded women because he had selected "penis". So his mother was excluded, as were Mrs. Cromwell, Miss Maples, Mrs. Wapole and her two little girls. The mailman had moved out of range by the time the Chronivac was activated. In the limited range, there was only one other male besides Jeff.
Looking down from his attic window was Harley. He was watching Jeff through his telescope while smoking weed, and jacking off. He thought the shirtless neighbor kid was hot.
As he watched Jeff type, he approached climax to his masturbation. As he peered through the scope, Jeff vanished. He felt his penis seem to grow in his hand, and then it jerked and exploded hot cum.
Jeff felt the giant hand gripping him, and felt the hot lava enema blast through him and out of his mouth. He stared upward in the dimly lit vaulted room, and recognized his neighbor as the owner of the new penis he had become.
"Hey, Jeff's gone and my dick feels different," said Harley, "Huh, I wonder if that means Jeff is now my dick." Stoner logic is correct this time.