Hi Chas - Good question. After the upgrade, the main URL structure changed so it is going to take Google a little while to RE-INDEX the site and update the URLs. This is a temporary issue that will go away as soon Google bots crawl the site and update the URLs.
What is happening is that Google still has the old site URL's indexed (those pages don't exist anymore) so when someone clicks on them, it defaults back to site homepage (better than an ugly 404 page error message).
The issue will go away by itself given some time. Thanks again Chas Chad