Google rewrites your title tags too.

Why it happens at all

Google's stated reasoning is that the title tag is written once for every possible query, while a result is shown for one specific query. A page targeting a broad subject may surface for a dozen different searches, and a generated headline can reflect what was actually asked in a way a fixed string cannot.

There is also a long history of title tags being unhelpful, either stuffed with keywords, padded with brand boilerplate, or left as whatever the CMS produced. A system that can substitute something clearer when the supplied version is poor produces better results, which is the same logic behind rewriting descriptions.

What usually triggers one

Length. A title long enough to truncate is a common cause of substitution, often by trimming the brand suffix or cutting to the informative part. Like descriptions, the limit is pixel width rather than a character count, which is why measuring in pixels beats counting to sixty.

Repetition and stuffing. The same phrase two or three times, or a chain of comma-separated keywords, reliably gets replaced by something plainer.

Boilerplate. Every page titled with the company name and a template suffix gives Google little to distinguish one from another, so it looks to the H1 or the page content for something specific.

A mismatch with the page. If the title promises something the content does not deliver, or the H1 says something clearly better, expect the H1 to win. This is worth reading as feedback: a rewrite that consistently prefers your H1 is telling you the H1 describes the page better than the title does.

Emptiness. No title tag, or a useless one such as Home or Untitled, and something will be generated for you, chosen without any input from you.

What to do about it

Write the title as the clearest possible description of the page, front-loading what it is actually about, and keep it short enough not to truncate. Make it match the H1 in substance rather than duplicating it word for word, and use the brand suffix where it earns the space rather than everywhere by default.

Then check reality. Search Console shows the queries a page appears for, and searching a few of them logged out shows what is actually being displayed. If the shown title is consistently better than yours, adopt it. If it is worse, the usual cause is one of the triggers above and it is fixable.

The part worth keeping in proportion

Rewrites are common, and they are not a problem to be defeated. There is no markup to force your version, and trying to game the substitution is effort against a system optimising for the searcher.

The productive framing is the one that applies to descriptions as well. Spend real effort on titles for the handful of commercial and head-term pages where the click is contested and valuable, write clear honest titles everywhere else, and accept that on a long tail of pages Google will assemble something query-specific and will often do it better than a single fixed string could.