Here's how it works.
You open a file, mull over the details then get hit with an idea and change it.
Previously you'd go Save As… and give it a sensible name "Blue version" because after some consideration and playing with the idea you went blue.
You now have 2 files, the original and the blue version. A single step.
Today you open a file, jump in your time machine to what you know you are going to do in the future, then Duplicate > Save > Give it the predictive name because you always work backwards from a name, then create. By this time let's hope you haven't forgotten what you were going to do, and actually do it, then let OSX save it automatically for you.
Oops, it has also saved the file you duplicated! Now did you, or did you not change anything in that, say half an hour ago or more? Can't quite remember? Better go and look through it in detail and compare it against your new document. Oh, dear in the process it now thinks you have changed something, even if you haven't and either saves or prompts you to. Now you don't even have the clue as to which was last saved to tell you where you were up to.
Or you foolishly went straight to your idea, because you were concentrating on just that, and made the changes.
Now what? Duplicate and Save to the new version name? Bugger, you'll then have 2 files, both with your changes in it. But you didn't want that. You wanted an original and a changed version, not two changed versions.
So you have to reopen the original file, in the hope of finding a reverted version and force it back to that and resave again. But are not quite sure it actually has.
As a bonus your older idea will now have the newer date.
Now you have totally lost track of where you were, and what you were doing. Never mind, give up.
Everybody is just Tweeting and posting selfies in Facebook!
Get with the program.
Peter