V3.15 on MacOS, long-time writer, first-time compiler, baffled by the output I’m getting. I’ve selected one Part with six chapters, selected “Treat compile group as complete manuscript”, and the result is based on previous versions of the selected chapters, plus some I didn’t select. I’m hoping there’s some fundamental thing I’m doing or not doing that someone will recognize. TIA.
I’m sorry, there is really nothing wrong in how you’ve described what you are doing. The devil is going to be in the details, no doubt. So far all we know about it is that you put water in the kettle, turned the stove up to medium, and somehow got soup instead of boiling water. 
I’d go back to the basics though. Select the same folder in the binder and hit ⌘1 to turn Scrivenings on. Is this fundamentally different from when you get when running a very simple “Default” settings compile of that very same folder? That’s what it sounds like you are saying, but I don’t understand how that would be possible.
In my mind, there are two likely possibilities:
A) You are opening an old version of the compiled output, while the new compiled output is going to another folder or filename.
B) You duplicated your own documents (possibly outside of the draft folder), edited the duplicate, but your compile setting is pointed at the old files in the same project.
For A, you can make Scrivener open the output for you, so as to eliminate the possibility that you’re selecting the wrong file to view.
For B… I’d suggest searching for a document title in your project that match one of the files shown on the right side of the compile window, and see if you find more than one version of that file.
Thanks very much for the quick replies! Taken together, they’ve enabled me to solve the problem.
The scrivenings were immaculate, and it was the first time I’d actually compiled, so there weren’t any old versions. But this made me look more closely at the output, and I realized it was actually just a single file full of notes and early draft pages that wasn’t marked as Include in compile. Yet it was the only thing being compiled. :-} So I moved it out of the folder and recompiled. Same result. I renamed it (it shared a couple of words with the files being compiled), and that did it. Thanks again, both of you! This is a HUGE relief.