"There was a trend toward papers in the private group being slightly older, although the reasons for this are not fully understood."
Might this be due to the papers in the private group containing a higher proportion available free of charge? This point could be relatively easily tested.
Private individuals noticing problematic data might do this away from any institutional resources. Journals tend to make papers available free of charge a year or two after publication. Also, private individuals complaining to journals might feel more confident that there was scientific misconduct if there was a history of problematic data.