The City guy returned to the drug testers within the hour, Ferdinand took 44 hours to get tested. Can't clear drugs out of your system in an hour, can in 44 hours. Think that's the main difference.
Ferdinand called the testers before they left Carrington (and before his "time ran out" so to speak, and the testers said "no can do". As I understand it, he didtry to get back but was refused.