Grossly oversimplifying it, but it is what my lawyer uncles say, and is consistent with what I saw as a juror on a civil trial involving many topics I'm personally an expert in, expert witnesses are paid to lie on the stand. At the very least they are paid very very well (the amount they were paid was emphasized at the trial I was at) to state only the information consistent with what the person paying them wanted to them to say, regardless of whether they know information that would be more helpful to the other side.