This link explains some of the findings:
The most interesting finding, in my opinion, is that 'approximately one third of the sisters whose brains were found to be riddled with Alzheimer's plaques and tangles at autopsy had shown no symptoms and scored normal results in all mental and physical tests while alive! Though the opposite result was true in other cases; such contradictory results show that there is much more to Alzheimer's than neurological changes in the brain alone.'
In other words, there were nuns whose post mortems indicated that they had the classic plaques and tangles which are assumed to indicate Alzheimer's yet they had no symptoms. Likewise there were nuns who did have symptoms but in whom, post mortem, there was no sign of the plaques and tangles.
And yet, years later, you can still read over and over again, apparently authoritative descriptions of the disease which make no mention of these findings!