This is cross-examination. Meaning, he is the defense’s witness. Meaning, he wants the defense to win. He feels that his findings support the defense, so he doesn’t want the prosecutor to lead the jury to improper inferences.

I will say that refusing to answer how many hours you worked does make you look like an unreliable witness. Like, the jury is going to find him less credible because of the way he is answering, and that will undermine the jury’s confidence in his findings.