I think a lot of it may have to do with the way he came into being a witness. He was not trained for this role and in reality it was a huge extra job that was dumped on him at short notice with no apparent time budget to do it alongside his "real" job.
I think the way it was put to him was that he needed to answer some specific questions from the legal team. And that is exactly what he did - answered the precise questions but didn't go beyond them.
Unlike some of the other professional expert witnesses we have seen in the inquiry - I don't think that he got any proper training as to what was required and indeed the lawyers who should have laid out expectations were among those pressuring him to change his testimony to benefit the PO