Comment on WordPress Site Title Tag SEO 2016 by Mark.
But you are right it might be something to do with Google. I have experienced wild gyrations in my Stats and it was showing that Google added site title to the search results if I had this clicked ON.
I noticed this because on one of my websites I have 4 WP Stallion theme installs on the same domain in different areas of the websites. Same domain, just set it up that way.
The search results were showing up with the root install’s title which was not thematically related to a deeper part of my website not related to my root’s title.
It seemed to be throwing relevancy off and hurting my rankings. That is how this whole discussion stated. In the end it is Google doing it I think. But when I switched Best SEO OFF, google only added this installs title rather than the root title. So it corrected it kind of, but still a mystery.
You might not notice this unless you had a website that had a multi level set up like mine which uses a few WP installs on the same site.