There's nothing odd about it, for the reasons explained. Well, no, but kind of.
See, technically, you're supposed to be able to reprint (and ONLY reprint) an uncopyrighted collection, regardless of the presence of copyrighted content. But no court is going to support you, because the collection is a derivative work (since it includes the original), which is covered under the original copyright. Plus, it's tacky and misleading to your consumers, since they can't reuse the reprint anywhere except in the same way.
So, no, nothing can be "partially in the public domain." However, you can have a work that's in the public domain has parts unusable due to copyright, which would functionally be the same thing.
Probably because of that and just plain old good nature, the rule here is to simply avoid the portions of books that are covered (or even probably covered) by other copyrights. And, in fact, I remember quite a few books that have been "sanitized" for posting here, though I can't recall which they were. Hopefully, there's a "this page is not included due to copyright" page inserted in their places, wherever they are.
The books I most worry about are the characters licensed from other media, where the renewals might not be available (films, for example--nobody has scanned'em, and my local library doesn't maintain a CCE collection) or where the copyright rules change drastically depending on where you live (radio dramas, for example--if you're in New York State, like I am, even Edison's first recording is protected under copyright until the 2030s, if I remember correctly).