WMG / The Secret Garden

Collin was misdiagnosed by a Con Man doctor.

Back when the book was set, doctors could be a lot shadier and sometimes misdiagnosed wealthy patients to get money. Seeing as the Cravens are wealthy, it's possible that when Collin was born, his father was on-edge since he was not only a preemie but what triggered the early labour was a fall. Thus, he sent for a doctor, but this doctor lied that Collin was in very poor condition when he wasn't, in order to make money off being the doctor of the wealthy family.