Cameron, as leader of the opposition at the time, employed a man, Andy Coulson, who had lost his job at News International following revelations of phone hacking. Cameron asked him whether he had any knowledge of phone "hacking", and Coulson assured him he didn't. Cameron took the view he should give the guy a second chance. He went on the be a very well respected communication director for the leader of the opposition, and then in government when Cameron won the election. There's no suggestion of wrong doing while working for Cameron at all.
Now it transpires Coulson was lying about knowing about phone "hacking" the PM has issued a full and comprehensive apology for his judgement in making that decision...
Compare this with Gordon Brown's apology when, a man working for him (Damian McBride), was caught making up lies to smear Brown's political opponents (including against David Cameron's wife, Samantha). Remember McBride was working for Brown at the time. His wrong-doing was related to the duties of the position (spin doctor) he held under his boss, Gordon Brown. Brown denied knowing anything about it. But the culture of poisonous spin aimed at undermining opponents (in and out of the Labour party) was well established by then. So, believe him if you will.
He sets out attempting to apologise but in the end, he utters the now infamous phrase "I take full responsibility for what happens. That's why the person responsible went immediately".