In some cases, public domain works are changed slightly for a particular edition. For instance, there is always a big wrangle over Jane Austen texts because various editions introduced what might be errors and might have been intentional changes and modern editors will attempt to "fix" them. You would not believe the arguments and discussions over the placement of a comma. However, such text fixups are part of the editorial process when an academic edits a text, not only writing notes and forwards. In such a case, the text is indeed copyrightable.
Some editions of Pride and Prejudice have a fairly large error that keeps being reproduced. In Vol. III, Ch. XV, a famous quote is changed: "For what do we live, but to make sport for our neighbours, and laugh at them in our turn?" is changed to "For what do we live, but to make sport
of our neighbours, and laugh at them in our turn?" It's one word, but changes the whole meaning of the sentence (and makes it illogical, in my view). A reader of my blog first noticed it, and
I wrote about it on the blog, and various readers pulled out their editions and found it reproduced in several other editions. But that's why it is sometimes valuable to re-edit public domain texts--and make them copyrightable.