if Mann is refusing to give the data for legal examination, it hardly looks as though he has nothing to hide , but there might be some other reason not disclosed

Indeed there might, or it may have been disclosed and not reported. As I indicated above, I do not know the details of this trial, but I have been in research where we would have had problems releasing information in court.   To give a simple example - which does not apply in this case, I freely admit - I am currently part of a cohort undergoing some medical trials. Releasing any data beyond the aggregated and anonymised would hit legal problems in its right because of Data Protection Act and other rights to privacy. In other cases, I have run research projects which had lots of non-disclosure agreements.

So simply being unable to think of the reason why this might be withheld is more a statement about our lack of imagination and/or the specific case. The line "If you are innocent you have nothing to hide" is a very dangerous one beloved by governments everywhere.