Ferdinand was charged with a different anti-doping infringement.
He was contacted by a tester and asked to provide a sample and then ran away rather than giving it. That refusal constitutes a doping violation in any sport.
Armitstead was charged not with refusing to give a sample, but with not being where she was supposed to be under the "whereabouts" system used as an additional anti-doping mechanism in some sports. Cyclists, among others, have to tell the anti-doping authorities where they will be for one hour of every day so as to facilitate easier Out of Competition testing. Armitstead was charged with not being available for testing in that hour three times over a one year period. Footballers can't be charged with this as football doesn't have a whereabouts system at all.
Built into the whereabouts system is an understanding that once in a while someone will not be available where and when they said they would be because of human error, medical emergencies etc. That's why you have to rack up three whereabouts issues for it to amount to a doping violation. That's entirely different from refusing to give a sample after being contacted. Armitstead got off because the Court of Arbitration for Sports decided that the first of her three whereabouts issues was down to the testing official rather than her. No three strikes, no ban.
I was a bit amused by the football journalists on twitter - who should know better - implying that Ferdinand had been harshly treated. Ferdinand somehow got a reduced suspension even though his case was open and shut. And no footballer is subject to the whereabouts system in the first place precisely because nobody wants to catch doping footballers and tarnish the most valuable product in sports.