I was selecting books that I'd recently acquired as part of T&T Clark Studies in Early Christianity. I had my library sorted by Last Updated column first, then I filtered it thus.
Then I clicked and selected one of the titles from the collection (probably the first, "Ancient Jewish and Christian Texts".
I did Shift+click on the Title column heading to add that as a secondary sort field.
Then I proceeded to look through all the other titles from the product description page of this collection, and Ctrl+click them all in the sequence in which they appeared there, until all of them were selected.
Result: The number of selected titles up at the upper right showed 12, but when I counted I saw that 13 were selected, matching the total number of books in the collection.
I was able to reproduce the discrepancy. Simplified to its bare essentials: select some titles first, then click a column heading to change the sort order, then Ctrl+click some more titles, and the number of titles that were selected before you changed the sort order will be missing from the total number at the upper right.
Logs attached from the first time I tried it.
