This has been a long-standing bug, but the introduction of the Last Updated column reminded me.
The bug is that there is no consistency as to whether sorting on a particular column will invoke a 'Group By' view.
It seems to me that the logic ought to be that were data is likely to be unique, then group by shouldn't be used (title is the obvious example), but where data is often likely to be common between resources, then it should be (author or publisher).
However that logic isn't applied to Rating, Tags, or Last Updated, which should all Group, to my way of thinking. A case could also be made for publication date, and electronic publication date.
An additional bug is that sorting by tags has no effect at all.