Why did Andy Pettitte’s testimony shock Roger Clemens’ prosecutors?

The desire of the federal government connected with the case of Roger Clemens was following: they desired to prove that this person had lied before the Congress about usage of anabolic steroids and HGH. But the government couldn’t achieve this goal. It was expected that Andy Pettitte would tell the jury that Roger Clemens had admitted that he had administered the banned medicines during a conversation in 1999 or in 2000. But the statement of Andy Pettitte was shocking for prosecutors. This man said that he couldn’t confirm that Roger Clemens had used steroids and HGH because he was not sure.
Actually, why were the prosecutors shocked by the affirmation of Andy Pettittte? This man swore for the 2008 Congressional hearings on application of steroids in the Major League Baseball that Roger Clemens acknowledged that he had taken HGH. But he pronounced opposite testimony at the Barrett Prettyman Federal Courthouse in Washington.
Michael Attanasio, a defense attorney for Roger Clemens, asked Andy Pettitte whether he may affirm that he is 50-50 that he has understood wrongly Roger Clemens about HGH. Andy Pettitte answered that he is indeed 50-50 about this aspect.
When Clemens’ prosecutors heard Pettitte’s answer, they didn’t know what to do. They confirmed that Andy Pettitte never answered this way, when he was asked several times.
The U. S. District Court Judge Reggie Walton claimed that he understood that Andy Pettitte’s answer has been conflicted. In fact, his statement was following: “I don’t know”.
The attorneys for Roger Clemens used the moment of the prosecutors’ confusion. They asked the judge to ignore Andy Pettitte’s testimony because he was 50-50. The attorneys concluded that he probably misunderstood Roger Clemens or he forgot the details of the conversation.  
The government has stated that the jury still allows taking into account the testimony of Andy Pettitte. ASUSA Dunham has confirmed that under federal rules jury is allowed to choose which affirmation to believe. He hopes that the jury will ignore the testimony that doesn’t support the government.
Thus, the testimony presented by Andy Pettitte and his wife led to problems. Testimonies of Andy’s wife during the first trial led to the declaration of a mistrial by the judge Walton.
Pettitte affirms that he is a friend of Roger Clemens. Clemens must also confirm that they are friends after Andy  Pettitte’s last testimony.