They waited seven days after doing the questions to Ender because they knew what kind of guy Ender was, so they needed proof of the victories in his battles for talking with him in order to know more about the strategies which had donde him win.
In the interview, Anderson and Graff found out that he had watched the videos of the insect wars to get training. And a important point to highlight is those videos weren't created with training purposes, only for advertising purposes. And other guys had been watching those videos but they didn't understand the strategies like Ender.
Graff and Anderson wanted to know, what Ender knew.
