Why would a multilingual site sometimes say "Save (this translation)" and other times say "Save (all translations)"? We have a multilingual Drupal 8 site supporting 17 different languages and many more localized versions of those languages.

I have been noticing that sometimes when I am editing a node it says "Save (this translation)" but then other times it says "Save (all translations)". 

What is triggering it to act differently at times? Have we configured something wrong with our translations/multilingual setup?

With what we are doing I believe we want them to save independently of each other.
The text of the submit button depends only on the translation settings of the status field. If the field "Publishing status" is not translatable for the content type, the button says "Save (all translations)".

You can change the language settings for content types, taxonomy vocabularies, user profiles here: /admin/config/regional/content-language