I have noticed that when I buy a leader who had 15 diligence on the recruitment screen after he enters my unassigned leaders tab the diligence changes to a lower value. This is misrepresentation by the job applicant. This seems to be a bug in supernova.
Same thing happened after I assigned and unassigned leader to be minister of finance the intelligence went down from 11 to 9. I guess its possible leader can lose skill over time but I think this is a bug.