Why were Roger Clemens’ prosecutors shocked by Andy Pettitte’s testimony?

The desire of the federal government connected with the case of Roger Clemens was following: they wanted to prove that this sportsperson had lied before the Congress about administration of anabolic steroids and HGH. But the government couldn’t achieve this goal. It was expected that Andy Pettitte would say that Roger Clemens had acknowledged that he had used the prohibited drugs during a conversation in 1999 or in 2000. But the statement of Andy Pettitte was shocking for prosecutors. He claimed that he couldn’t affirm 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 intake of steroids in the Major League Baseball that Roger Clemens admitted to intake of HGH. But he pronounced opposite statements at the Barrett Prettyman Federal Courthouse in Washington.
Michael Attanasio, a defense attorney for Roger Clemens, asked Andy Pettitte whether he can say that he is 50-50 that he has understood wrongly Roger Clemens about HGH. Pettitte answered that he is really 50-50 about this aspect.
When Clemens’ prosecutors heard the answer of Andy Pettitte, they didn’t know what to do. They affirmed that Andy Pettitte never said this way, when he was asked several times.
The judge Reggie Walton noticed that he understood that Andy Pettitte’s answer has been conflicted. Actually, his testimony was following: “I don’t know”.
The attorneys for Roger Clemens used the moment of the prosecutors’ confusion. They asked the judge to brush aside Andy Pettitte’s testimony because he was 50-50. According to the attorneys, he probably misunderstood the athlete Roger Clemens or he can’t remember the details of the conversation.  
The government has stated that the jury still allows considering the testimony of Andy Pettitte. ASUSA Dunham has pointed out that under federal laws jury is allowed to choose which affirmation to believe. He hopes that the jury will disregard the testimony which doesn’t support the government.
So, the testimony presented by Andy Pettitte and his wife caused problems. Testimonies of Andy’s wife during the first trial resulted in the declaration of a mistrial by the judge Walton.
Pettitte affirms that he is Roger Clemens’ friend. Clemens must also confirm that they are friends after Andy  Pettitte’s last testimony.