When the page title in SERPs is changed?

Hi all,

Recently I've seen some of my posts titles are changed in SERPs.

Its a wordpress with yoast plugin. In meta & titles -> post section I have only %title% for the post titles.

Google adds to the page title a dash and something related to my website. Looks like the site_title but its not exactly that.

And when I look at the page source code, that added content after the dash is not found anywhere.

If I set the title in the yoast seo box below the post editor, there is no change.

Why titles are changed?

My real question is

if its possible to know when google uses the page <title> tag for the SERPs title (blue link) and when doesn't

When <title> is not used. How they build the SERPs title? Randomly from page content?

They usually do it when you have a spammy bad title. Try changing it to something more short and accurate without any keyword stuffing.

google uses what it wants ....mostly the title but sometimes they use some of the text as well. no way to control that 100%